Solar module structures and assembly methods for three-dimensional thin-film solar cells

ABSTRACT

Solar module structures  210  and  270  and methods for assembling solar module structures. The solar module structures  210  and  270  comprise three-dimensional thin-film solar cells  110  arranged in solar module structures  210  and  270 . The three-dimensional thin-film solar cell comprises a three-dimensional thin-film solar cell substrate ( 124  and  122 , respectively) with emitter junction regions  1352  and doped base regions  1360 . The three-dimensional thin-film solar cell further includes emitter metallization regions and base metallization regions. The 3-D TFSC substrate comprises a plurality of single-aperture or dual-aperture unit cells. The solar module structures  270  using three-dimensional thin-film solar cells comprising three-dimensional thin-film solar cell substrates with a plurality of dual-aperture unit cells may be used in solar glass applications. The solar module structures  210  using three-dimensional thin-film solar cells comprising three-dimensional thin-film solar cell substrates with a plurality of single-aperture unit cells may be used in building façade and rooftop installation applications as well as for centralized solar electricity generation.

This application claims the benefit of provisional patent applications60/828,678 filed on Oct. 9, 2006 and 60/886,303 filed on Jan. 24, 2007,which are hereby incorporated by reference.

FIELD

This disclosure relates in general to the field of photovoltaics andsolar cells, and more particularly to solar module structures andassembly methods. Even more particularly, the presently disclosedsubject matter relates to solar module structures and assembly methodsfor three-dimensional (3-D) thin-film solar cells (TFSCs).

DESCRIPTION OF THE RELATED ART

Renewable, high-efficiency, and cost-effective sources of energy arebecoming a growing need on a global scale. Increasingly expensive,unreliable, and environmentally-risky fossil fuels and a rising globaldemand for energy, including electricity, have created the need foralternate, secure, clean, widely available, cost-effective,environmentally-friendly, and renewable forms of energy. Solarphotovoltaic (PV) electricity generation using solar cells is uniquelysuited to meet the needs of residential, commercial, industrial, andcentralized utility applications. Key attributes that make solar energyattractive are the abundant, worldwide, point-of-use supply of sunlight,environmental friendliness, scalability (from milliwatts to megawatts),secure point-of-use generation of solar electricity, and excellentdistributed energy economics. The sun provides more energy to the earthin one hour than the annual energy consumption of the entire world. Muchof the earth's surface receives a significant amount of annual sun-hourswhich may be effectively harnessed for clean and secure electricitygeneration. A key driver for this market pull is a rising publicawareness of environmentally-benign technologies. However, due torelatively low solar cell efficiencies (e.g., less than 12% for mostthin-film technologies and roughly 12% to 18% for most crystallinesilicon solar cell technologies), high costs of raw materials (e.g.,silicon for crystalline silicon wafer solar cells) and manufacturingprocesses, limitations on cost-effective and efficient electricalstorage, and a general lack of infrastructure to support solar cellproliferation, to date there has been limited use of this energysolution (currently, electricity generation by solar photovoltaicsaccounts for less than 0.1% of total worldwide electricity generation).

For commercial applications, cost of energy to the end-user (e.g., incents/kWh for electricity) should be sufficiently low and comparable toor even better than that from utility grids using conventionalelectricity generation sources. The solar photovoltaic electricitygeneration, which currently accounts for less than 0.1% of the globalelectricity generation, may be substantially expanded if it achievescost parity with conventional grid electricity. As the costs of solarcells and modules (typically expressed as $/W_(p)) are reduced,grid-tied solar photovoltaic applications are gaining acceptance at anaccelerated pace, making them an attractive option for significantproliferation in electricity generation.

In the price-sensitive solar cell market, two principal technologyoptions exist. On the one hand, crystalline silicon (c-Si) wafers mayserve as the basis for solar cell formation (currently accounting formore than 90% of the solar PV market). On the other hand, thin-film(amorphous and polycrystalline) technologies using silicon and othersemiconductor absorber materials (such as amorphous silicon, CdTe, orCIGS) may offer significant cost advantages compared to crystallinesilicon wafer-based solar cells. These different approaches are atopposite ends of the price-performance scale. Crystalline silicon wafersoffer higher performance, but at higher costs (due to the relativelyhigh cost of starting monocrystalline and multicrystalline siliconwafers). Thin-film technologies may offer lower manufacturing costs, buttypically at lower performance levels (i.e., lower efficiencies). Forboth approaches, the price-per-watt typically increases as cellefficiencies rise (due to higher material and/or manufacturing costs).

Due to a rapid annual growth rate of more than 40% during the past tenyears and the concurrent demands for silicon material by bothsemiconductor microelectronics and solar PV industries, the solar PVindustry has been experiencing a shortage of polysilicon feedstocksupply. The polysilicon feedstock shortage has significantly constrainedthe solar PV industry growth, particularly during the past severalyears. In fact, the solar cell industry currently consumes over half ofthe worldwide production of high-purity polysilicon feedstock. Withinthe last few years, the contract price of polysilicon has increased fromroughly $30/kg to roughly $85/kg, with spot prices exceeding $250/kg.This has led to large increases in the price of monocrystalline andmulticrystalline silicon wafers, which now account for roughly half ofthe total solar module manufacturing cost.

The trend in the mainstream crystalline silicon (c-Si) wafer solar cellindustry has been to scale down wafer thicknesses to below 200 microns(in order to reduce the amount of silicon material in grams used perwatt of solar cell rated peak power). For example, monocrystallinesilicon wafer solar cells are projected to scale down to a thickness ofroughly 120 microns by 2012, from a current wafer thickness of roughly200 microns. Multicrystalline silicon wafer solar cells are projected toscale down to a thickness of roughly 180 microns by 2012, from a currentaverage wafer thickness of roughly 260 microns. This wafer thicknessreduction, however, presents additional challenges related to mechanicalrigidity, manufacturing yield, and solar cell efficiency. Despite itshigh cost, crystalline silicon (c-Si) technology still dominates thesolar cell market, mainly due to higher efficiencies and synergies withthe established microelectronics industry and supply chain. Currently,c-Si accounts for slightly over 90% of the solar cell market (95% whenribbon silicon is included).

Historically, crystalline silicon solar cells have achieved a 20% costreduction for each doubling of cumulative global cell production(measured in megawatts or MW_(p) and gigawatts or GW_(p)). It isprojected that through innovative cost reduction and efficiencyenhancement methods, the cost of electricity derived from grid-connectedrooftop solar photovoltaic modules may become comparable to the cost ofelectricity purchased from the utility grid in five to ten years. A 2005survey of the commercially available monocrystalline silicon andmulticrystalline silicon solar modules reports the solar moduleefficiencies then in the range of 9.1% to 16.1%, with a medianefficiency value of about 12.5%. Commercial crystalline silicon modulesusually show a rapid initial efficiency degradation of 1% to 3%(relative) due to various effects, including photodegradation effects inwafered solar cells (e.g., wafer minority carrier lifetime degradation).Monocrystalline silicon wafer solar cell efficiencies are projected toincrease to roughly 20.5% by 2012, from a current efficiency of roughly16.5% (leading-edge commercially available monocrystalline silicon solarcell and solar module efficiencies are currently about 21.5% and 18%,respectively). Multicrystalline silicon wafer solar cell efficienciesare projected to increase to roughly 18% by 2012, from a currentefficiency level of roughly 15.5%.

State-of-the-art crystalline silicon solar cell manufacturing currentlyuses about 10 grams of high-purity polysilicon feedstock per peak watt(g/W_(p)), resulting in a polysilicon feedstock material cost of about$0.85/W_(p) (assuming a polysilicon price of $85/kg). Over the next fiveyears, the projected trends of solar cell wafer thickness reduction(e.g., to less than 200 micron wafers) and a long-term assumed price ofabout $20/kg for solar-grade polysilicon may reduce the polysiliconfeedstock cost (in g/W_(p)) by about a factor of four to eight to about$0.10/W_(p) to $0.20/W_(p). Thus, any competing solar cell technologiesshould benchmark their manufacturing cost goals against this reduced rawmaterial cost number. For a given cell efficiency, silicon waferthickness reduction presents a prime opportunity for solar cell costreduction by reducing the amount of polysilicon feedstock consumed perwatt of peak solar power.

The cost associated with wire saws, amounting to about $0.25/W_(p) forcurrent silicon solar cells provides another wafer-related costcomponent for silicon wafer solar cells. Innovative and cost-effectivetechnologies that eliminate the kerf losses associated with sawing andslicing should further facilitate silicon solar cell cost reductions. Itis projected that the wafer-based crystalline silicon solar modulemanufacturing cost (which is currently on the order of $2.10 per watt tomore than $2.70 per watt) may be reduced to the range of roughly$1.50/W_(p) to $1.80/W_(p) by the year 2012, in part due to wafer sawingkerf loss reduction to roughly 130 microns by 2012 from the currentvalue of roughly 200 microns. The overall cost reductions forwafer-based crystalline silicon solar cells may come from varioussources including: lower cost polysilicon feedstock, thinner wafers,higher cell-level efficiencies, reduced wafer sawing kerf losses, andincreased economy of scale or manufacturing volume.

State-of-the-art silicon wafer solar cell fabrication facilities (“solarfabs”) typically produce 125 mm×125 mm up to 156 mm×156 mm solar cellstoday. The trend in crystalline silicon wafer solar cells is towardthinner and larger wafers. The monocrystalline and cast (as well asribbon) multicrystalline silicon solar cell wafer thicknesses inleading-edge solar cells used for power generation modules are projectedto be reduced to around 150 and 200 microns, respectively, by around2009-2010. Any cost-effective, high-efficiency, innovative silicon solarcell technology which enables a substantial reduction of the siliconmaterial consumption (e.g., wafer or film thickness) per W_(p) of cellpower compared to the above-mentioned current and projected 2009-2010numbers may offer significant promise as a viable commercial solar celltechnology for solar photovoltaic applications (e.g., residential,commercial, and industrial rooftop as well as large-scale centralizedutilities electrical power generation applications).

Higher solar cell efficiencies have favorable effects on the entiresolar cell value chain and levelized cost of energy (LCOE in $/kWh) dueto reduced material consumption and cost as well as reducedbalance-of-system (BOS) costs (e.g., area-related solar moduleinstallation and inverter costs). The current mainstream commercialcrystalline solar cells provide efficiencies on the order of 14% to 17%.It is expected that the projected crystalline silicon solar cellefficiencies in commercial solar cells may approach around 19% and 17%for monocrystalline and multicrystalline silicon solar cells,respectively, by the year 2009. A key area for new solar cell businessopportunities is development of innovative cell structures andsimplified process flows which may drive efficiencies up while loweringoverall solar cell and module manufacturing costs. For alternative(e.g., thin-film PV) approaches to succeed over the mainstreamwafer-based crystalline silicon solar cell technologies, they shouldprovide higher efficiencies at even lower manufacturing costs comparedto the projected efficiency and cost numbers for the mainstreamwafer-based crystalline silicon solar cells when the new technology isfully commercialized.

Economy-of-scale fab cost reduction associated with high-volume solarfab capacities is a key factor impacting LCOE. The state-of-the-arthigh-volume solar photovoltaic fabs have annual production capacities onthe order of or in excess of 50 MW_(p) to 100 MW_(p) (MW_(p)=1 millionW_(p)). High-volume solar photovoltaic fab capacities are expected toincrease substantially to annual production rates of several hundredMW_(p) or even approaching 1 GW_(p) (GW_(p)=1 billion W_(p)) in thecoming decade. While very-high-volume solar fabs in the range of 100MW_(p) to 1 GW_(p) should facilitate longer term cost reductions(including LCOE) through high-volume manufacturing economies of scale,the relatively high initial fab investment costs, which may easilyexceed $100M, may impose certain limits on solar photovoltaic fabconstruction options. Ideally, the preference may be to developinnovative crystalline silicon solar cell designs and simplifiedmanufacturing processes which facilitate substantial manufacturing costreductions in solar cells and modules even in smaller-scale (and lesscapital intensive) fabs with modest production volumes (e.g., annualproduction volumes in the range of 5 MW_(p) to 50 MW_(p)). This type oftechnology would allow for modest-volume solar photovoltaic fabs withmodest fab setup and operation costs. Reduced fab setup and operationcosts would further facilitate global proliferation of cost-effectivesolar modules, enabling construction of a multitude of very affordablemodest-volume fabs (in contrast to having to set up very expensivehigh-volume fabs in order to achieve sufficient economy of scale formanufacturing cost reduction). Of course, an innovative solar celltechnology that meets the above-mentioned criteria for cost-effective,modest-volume fabs (i.e., meeting the LCOE roadmap requirements even atmodest production volumes in low-cost fabs set up for simplified solarcell processing), may also be applicable to very-high-volume (e.g.,greater than 100 MW_(p)) solar fabs. Such solar photovoltaic fabs cantake further advantage of the economies of scale associated withincreased volume.

Thin-film solar cell (TFSC) technologies (e.g., amorphous silicon, CdTe,and CIGS) require little absorber material (usually much less than 10microns in thickness) to absorb typical standard “Air Mass 1.5” (AM-1.5)solar illumination due to absorption bands that are well matched to thesolar spectrum. The TFSC absorber material may be deposited oninexpensive substrates such as glass or flexible metallic ornon-metallic substrates. TFSCs typically offer low cost, reduced moduleweight, reduced materials consumption, and a capability for usingflexible substrates, but are usually much lower in efficiency (e.g.,usually 5% to 12%). In the case of prior art thin crystalline siliconfilms, there are a number of major problems and challenges with the useof flat silicon films (such as epitaxially growth silicon films withthicknesses below 50 microns) for low-cost, high-performance solarcells. These include: relatively low solar module efficiencies(typically 7% to 12%), field degradation of module efficiencies, scarceand expensive absorber materials (e.g., In and Se for CIGS and Te forCdTe), limited validation of system field reliability, and adverseenvironmental impact of non-silicon technologies such as CIS/CIGS andCdTe.

Prior art FIG. 1 shows process flow 10 for fabricating c-Si TFSCs usingplanar silicon thin-film absorber layers produced by epitaxial silicon.This prior art TFSC fabrication process flow uses several shadow maskprocess steps to form the cell structure. The cell absorber is simply athin planar film of c-Si formed by silicon epitaxial growth processing.The cell uses frontside silicon texturing to improve light trapping anda detached rear aluminum mirror to improve the cell efficiency. Step 12starts with single-crystal p⁺ CZ silicon. Step 14 involveselectrochemical HF etching of silicon to form 2-layer porous siliconcomprising a 1 micron top layer with 20% porosity and a 200 nanometerrear layer with greater than 50% porosity. Step 16 involves a hydrogen(H₂) anneal at 1100° C. for 30 minutes. Step 18 involves epitaxialsilicon growth at 1100° C. using trichlorosilane or SiHCl₃ (depositionrate of 1 micron per minute), forming 2 microns of p⁺⁻Si and 30 micronsof p-Si. Step 20 involves frontside surface texturing by wet KOH etchingto form upright surface pyramids. Step 22 involves the first shadow maskprocess, with LPCVD silicon nitride (SiN_(x)) deposition through ashadow mask to define emitter diffusion windows. Step 24 involves solidsource phosphorus diffusion at 830° C. (to achieve 80 Ω/square for then⁺ doped junction). Step 26 involves the second shadow mask process,with frontside metallization (titanium/Pd/silver grid) by evaporationthrough shadow mask. Step 28 involves emitter surface passivation byhydrogenated PVD or PECVD SiN_(x). Step 30 involves contact frontsidebusbar by a conductive adhesive. Step 32 involves gluing the cellfrontside to MgF₂-coated glass using clear glue. Step 34 involvesseparating the cell from silicon wafer by mechanical stress. Step 36involves the third shadow mask process, with backside aluminummetallization using evaporation through shadow mask. Finally, step 38involves attaching an aluminum reflector at 200 micron spacing from thecell backside.

Prior art FIG. 2 shows another process flow method 40 for fabrication ofsolar cells on silicon wafers with self-aligned selective emitter andmetallization. This prior art process uses laser processing to patternthe top cell dielectric layer while melting the underlying silicon toform the heavily-doped n⁺⁺ emitter contact diffusion regions (afterformation of the lightly diffused selective emitter regions by rapidthermal annealing). Step 42 starts with single-crystal p-type silicon.Step 44 involves saw damage removal etch and anisotropic texturing etchin dilute NaOH at 90° C. Step 46 involves spin-on application and dryingof phosphorus diffusion source. Step 48 involves rapid thermal annealingto form lightly diffused emitter (80 to 200 Ω/square). Step 50 involvesapplication of backside metal contact by vacuum evaporation or screenprinting of aluminum or silver/aluminum alloy, followed by drying. Step52 involves backside metal sintering/firing (e.g., at 820° C. inoxygen/nitrogen) for a screen-printed contact (fires the metal pastewhile oxidizing the dielectric to raise its resistance to the metalplating solution). Step 54 involves laser processing to pattern the topdielectric layer while melting the underlying silicon to form the n⁺⁺contact diffusion region. Step 56 involves dilute HF etch to preparemetal plating surface. Step 58 involves electroless nickel plating at90° C. for five minutes. Step 60 involves nickel sintering at 350° C. to450° C. (in nitrogen, argon, or forming gas). Step 62 involves anadditional 2 minutes of nickel plating followed by long electrolesscopper plating to form thick high-conductivity copper film. Step 64involves flash immersion silver (silver) deposition on copper surface.Finally, step 66 involves edged junction isolation (e.g., using lasergrooving, edge cleavage, or plasma etching).

With regard to the prior art crystalline silicon (c-Si) thin-film solarcell (TFSC) technology, there are difficulties associated withsufficient surface texturing of the thin silicon film to reduce surfacereflectance losses, while reducing the crystalline silicon filmthickness. This places a limit on the minimum flat (co-planar)monocrystalline silicon thickness due to production yield and cellperformance (efficiency) considerations. In the case of a flat orco-planar film, it is essential to use surface texturing since thereflectance of an untextured crystalline silicon film is quite excessive(can be greater than 30%) and results in substantial optical reflectionlosses and degradation of the external quantum efficiency. Thus,reduction of reflectance-induced photon losses in co-planar epitaxialsilicon films requires effective surface texturing which itself places alimit on the minimum epitaxial silicon layer thickness. Depending on thefilm surface texturing requirements and processes, the minimumcrystalline silicon layer thickness may be on the order of at least 10microns (so that the texturing process does not break through anyportions of the crystalline silicon layer).

In addition, substantially reduced mean optical path lengths in thinplanar crystalline silicon films result in reduced photon absorption,particularly for photons with energies near the infrared bandgap ofsilicon (800 to 1100 nanometers), resulting in reduced solar cellquantum efficiency (reduced short-circuit current or J_(sc)). Thisresults in serious degradation of the solar cell efficiency due toreduced cell quantum efficiency and reduced J_(sc). For instance, in aco-planar (flat) crystalline silicon absorber layer with thickness of 20microns, a solar light beam impacting the cell at a near-normal anglewould have an effective path length equal to the film thickness, far tooshort for the solar radiation photons with energies near the infraredbandgap of silicon (i.e., with wavelengths of roughly 800 to 1100nanometers) to be absorbed effectively in the silicon thin film. Infact, a reduction of the active cell silicon thickness to below roughly50 microns results in appreciable reduction of J_(sc) and the resultingsolar cell efficiency, with this degradation effect rapidly acceleratingwhen the silicon film thickness is reduced below roughly 20 microns.Thus, a co-planar thin crystalline silicon film may also requireeffective light trapping using both top surface texturing and rearsurface back reflection of the light exiting the back surface of thecrystalline silicon film in order to create effective optical pathlengths equal to a large multiple of the crystalline silicon filmthickness.

The prior art technologies using this approach mostly use either backreflection through internal reflection of the light at the crystallinesilicon film/silicon substrate, or reflection from a blanket backsidecontact (such as a back surface field aluminum contact/mirror). The backreflectance provided by these techniques may not be great (e.g., roughly70% effective near-IR rear reflectance), constraining the performancegain that would have otherwise been achieved by an optimal backreflector. The problem with this approach is that the primary incidentbeam always passes the crystalline silicon film only once. Anysubsequent second passes of the primary incident beam photons aredependent on the back surface reflection.

There is also the problem of lack of rigidity and mechanical support ofthe thin film during cell and module processing steps. This problemrelates to the mechanical strength of a large-area (e.g., 200 mm×200 mm)thin silicon film. It is well known that reducing the large-areacrystalline silicon wafer thickness to below 100 microns results in asubstantial loss of TFSC substrate mechanical strength/rigidity, andsuch thin wafers tend to be flexible and very difficult to handlewithout breakage during cell fabrication process flow.

Large-area, co-planar (flat) crystalline silicon films thinner than, forinstance, 50 microns must be properly mounted and supported on acost-effective support or handle substrate in order to achieveacceptable yield for solar cell and module manufacturing. One approachis to grow and retain the thin epitaxial film on a relatively low-cost(e.g., metallurgical-grade) silicon substrate (over which the epitaxiallayer is grown); however, this approach suffers from some inherentproblems constraining the ultimate solar cell efficiency. Anotherapproach is to release or lift off the epitaxial silicon film from its(reusable) parent silicon substrate and subsequently place it on acheaper non-silicon support or handle substrate to provide mechanicalstrength through the solar cell process flow. This approach may sufferfrom any thermal coefficient of expansion (TCE) mismatch between thesupport/handle substrate and silicon film during any high-temperatureoxidation and anneal processes, as well as potential contamination ofthe thin epitaxial silicon film from the non-silicon support substrate(both creating possible manufacturing yield and performance/efficiencydegradation problems).

The cost of the monocrystalline silicon film growth process usingsilicon epitaxy, particularly for thicker epitaxial films withthicknesses in excess of 30 microns is an additional issue which shouldbe addressed. Using a relatively small epitaxial film thickness (in oneembodiment, much below 30 microns) may lower the cost of epitaxy to anattractive range. However, this would present various challenges forfabrication of planar silicon thin-film solar cells. As stated, thinnerco-planar (flat) epitaxial films (e.g., in the range of much less than30 microns) produce a number of problems and challenges, including alack of film mechanical strength, constraints limiting effective surfacetexturing of thin silicon films for low surface reflectance and reducedoptical reflectance losses, relatively short optical path lengths, andreduced cell quantum efficiencies. Effective light trapping is essentialfor enhanced thin-film c-Si solar cell efficiencies. The requirement foreffective light trapping is based on a combination of front surfacetexturing and back surface mirror, while achieving sufficiently lowsurface recombination velocities (for high cell efficiencies). This isvery difficult to achieve in the co-planar (flat) c-Si thin film solarcells.

High-performance c-Si thin-film solar cells require some patterningsteps or patterned processing steps (e.g., for formation of selectiveemitter, frontside emitter or backside emitter wrap-throughmetallization contacts, backside base metallization contacts, etc.).These patterning steps are usually achieved using photolithography,screen printing, and/or shadow-mask deposition (e.g., shadow-masksputtering or evaporation) processes. The use of photolithography and/orscreen printing and/or shadow-mask deposition patterning steps usuallyincreases the manufacturing process flow complexity and cost, and mayalso detrimentally impact the fabrication yield as well as the ultimateachievable solar cell efficiency.

Therefore a need has arisen for a thin-film solar cell (TFSC) whichcorrects the problems identified above.

Yet a further need exists to address shortcomings of existing mainstreamc-Si solar cell technology. This includes reducing the amount ofpolysilicon feedstock consumed per peak watt of solar power, andeliminating the kerf losses associated with sawing and slicing; thus,substantially reducing the overall solar cell manufacturing cost.

A further need exists for innovative solar cell structures andsimplified process flows, increasing cell and module efficiencies whilesignificantly lowering the overall solar cell and module manufacturingcosts. A still further need exists for innovative c-Si solar celldesigns and simplified self-aligned manufacturing processes whichfacilitate substantial solar cell and module cost reduction even in fabswith modest production volumes, enabling low to mid-volume solar cellfabs with modest fab setup and operation costs (thus, achievingeconomies of scale for manufacturing cost reduction at substantiallylower fab volumes than the prior art fabs).

A still further need exists to address shortcomings of existing TFSCtechnology. This includes addressing difficulties associated withsufficient surface texturing of the thin planar silicon films to reducesurface reflectance losses, which currently places a limit on theminimum flat (co-planar) crystalline silicon thickness due to productionyield and cell performance considerations. A still further need existsfor effective light trapping based on a combination of front surfacetexturing and back surface mirror, while achieving low surfacerecombination velocities (for high cell efficiencies).

A still further need exists to address additional shortcomings ofexisting TFSC technologies. This includes the problem of lack ofrigidity and mechanical support of the thin film substrate during celland module processing steps, thus, necessitating the use of support orhandle substrates (made of silicon or another material) for thethin-film TFSC substrates. This further includes the cost of theepitaxial silicon film growth process, particularly for thickerepitaxial films required for planar crystalline silicon TFSCs. Thisfurther includes the requirement of multiple photolithography and/orscreen printing and/or shadow-mask processing/patterning steps whichusually increase the manufacturing process flow complexity and cost, andmay also detrimentally impact the fabrication yield as well as theultimate achievable solar cell efficiency.

SUMMARY

In accordance with the present disclosure, solar module structuresutilizing three-dimensional thin-film solar cells (3-D TFSCs) andmethods of assembling such solar module structures are provided. Thesolar module structures of the disclosed subject matter utilizing 3-DTFSCs substantially eliminate or reduce disadvantages and problemsassociated with previously developed TFSCs, both in terms of efficiencyand manufacturing cost.

According to one aspect of the disclosed subject matter, there isprovided a solar module structure comprising a top protective layer, aplurality of 3-D TFSCs, a printed circuit boar, and a protective backplate. Each 3-D TFSC comprises a 3-D TFSC substrate comprising emitterand base regions, and where the 3-D TFSC substrate comprises a pluralityof unit cells.

According to another aspect of the disclosed subject matter, there isprovided a solar module structure comprising a top glass plate, aplurality of 3-D TFSCs, and a bottom glass plate. Each 3-D TFSCcomprises a 3-D TFSC substrate comprising emitter and base regions, andwhere the 3-D TFSC substrate comprises a plurality of unit cells.

According to still another aspect of the disclosed subject matter, thereis provided a solar module structure comprising a top protective layer,a plurality of 3-D TFSCs, and a bottom protective layer. Each 3-D TFSCcomprises a 3-D TFSC substrate comprising emitter and base regions, andwhere the 3-D TFSC substrate comprises a plurality of unit cells.

According to still another aspect of the disclosed subject matter, thereis a provided a solar module structure. The solar module structurecomprises a top encapsulant layer, a plurality of 3-D TFSCs, a printedcircuit board (PCB), a rear encapsulant layer and a protective backplate. The 3-D TFSC comprises a 3-D TFSC substrate with emitter junctionregions doped base regions. The 3-D TFSC further includes emittermetallization regions and base metallization regions. The 3-D TFSCsubstrate comprises a plurality of single-aperture unit cells.

According to still another aspect of the disclosed subject matter, thereis a provided a method for assembling a solar module structure. Themethod includes a first step of patterning a frontside and a backside ofa double-sided printed circuit board coated with metallic foilsaccording to desired frontside and backside interconnect layouts. Themethod includes a next step of applying a first coating layer to therear side of a plurality of 3-D TFSCs. The 3-D TFSC comprises a 3-D TFSCsubstrate with emitter junction regions doped base regions. The 3-D TFSCfurther includes emitter metallization regions and base metallizationregions. The 3-D TFSC substrate comprises a plurality of single-apertureunit cells. The method includes a next step of placing the plurality of3-D TFSCs on the frontside of the double-sided printed circuit board.The method includes a next step of preparing a solar module assembly.The solar module assembly includes a glass layer, a top encapsulantlayer, the plurality of 3-D TFSCs on the frontside of the double-sidedprinted circuit board, a rear encapsulant layer, and a protective backplate. Finally, the method includes sealing and packaging the solarmodule assembly.

According to still another aspect of the disclosed subject matter, thereis a provided a solar module structure. The solar module structurecomprises a top glass plate, a plurality of 3-D TFSCs, a rear patternedcell interconnect layer, and a bottom glass plate. The 3-D TFSCcomprises a 3-D TFSC substrate with emitter junction regions doped baseregions. The 3-D TFSC further includes emitter metallization regions andbase metallization regions. The 3-D TFSC substrate comprises a pluralityof dual-aperture unit cells.

According to still another aspect of the disclosed subject matter, thereis a provided a method for assembling a solar module structure. Themethod includes a first step of applying a rear patterned cellinterconnect layer to the rear side of a 3-D TFSC. The 3-D TFSCcomprises a 3-D TFSC substrate with emitter junction regions doped baseregions. The 3-D TFSC further includes emitter metallization regions andbase metallization regions. The 3-D TFSC substrate comprises a pluralityof dual-aperture unit cells. The method includes a next step ofpreparing a solar module assembly. The solar module assembly includes atop glass plate, the plurality of 3-D TFSCs with the rear patterned cellinterconnect layer applied to the rear side, and a bottom glass plate.Finally, the method includes sealing and packaging the solar moduleassembly.

These and other advantages of the disclosed subject matter, as well asadditional novel features, will be apparent from the descriptionprovided herein. The intent of this summary is not to be a comprehensivedescription of the claimed subject matter, but rather to provide a shortoverview of some of the subject matter's functionality. Other systems,methods, features and advantages here provided will become apparent toone with skill in the art upon examination of the following FIGURES anddetailed description. It is intended that all such additional systems,methods, features and advantages be included within this description, bewithin the scope of the accompanying claims.

BRIEF DESCRIPTIONS OF THE DRAWINGS

The features, nature, and advantages of the disclosed subject matter maybecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

FIG. 1 (PRIOR ART) shows a prior art process flow for fabricatingcrystalline silicon (c-Si) thin-film solar cells (TFSCs) using planarsilicon thin-film absorber layers produced by silicon epitaxy;

FIG. 2 (PRIOR ART) shows a prior art process flow for fabrication ofsolar cells on silicon wafers including self-aligned selective emitterand metallization;

FIG. 3 (PRIOR ART) summarizes the key process steps eliminated by thecurrent disclosure, compared to the prior art;

FIG. 4 summarizes the high-level process flow and the competitiveadvantages of the current disclosure, compared to the prior art;

FIG. 5 provides another summary of the key features and benefits of thecurrent disclosure;

FIG. 6 shows a view of series connections of TFSCs in a solar moduleassembly;

FIG. 7 shows a top view of an embodiment of a hexagonal-prism TFSCincluding a square-shaped hexagonal-prism 3-D TFSC substrate with aplanar peripheral silicon frame;

FIGS. 8A and 8B show scanning electron microscopic views of twoembodiments of a hexagonal-prism 3-D TFSC, without and with a rear baselayer, respectively (FIGS. 8A and 8B show the dual-aperture andsingle-aperture 3-D TFSC substrates, respectively);

FIG. 9 provides an overview of the 3-D TFSC substrate and solar cellfabrication process flow;

FIG. 10 shows a view of an embodiment of a template including hexagonalprism posts;

FIGS. 11A and 11B show magnified scanning views (with two differentmagnifications) of one embodiment of a template including hexagonalprism posts;

FIG. 12 shows a view of an embodiment of a template including staggered(shifted) square prism posts;

FIG. 13 shows a view of the frontside metallization pattern of aprinted-circuit board (PCB) used for solar module assembly using theTFSCs of the current disclosure;

FIGS. 14 and 15 show views of the backside metallization pattern of aPCB used for solar module assembly using the TFSCs of the currentdisclosure;

FIG. 16A shows an enlarged top view of the frontside of a solar modulePCB, showing one of the PCB patterned metallization sites for placementof one of the solar cells of the current disclosure;

FIG. 16B shows an enlarged top view of the backside of a solar modulePCB, showing the series connections of the adjacent cells on the PCB;

FIG. 17 shows a cross-sectional view of an embodiment of a solar modulestructure comprising the TFSCs of the current disclosure and a temperedglass cover;

FIG. 18 shows an embodiment of a process flow for fabrication of solarmodules using a tempered glass cover;

FIG. 19 shows a cross-sectional view of an embodiment of a solar modulestructure comprising the TFSCs of the current disclosure and a coatedlayer cover;

FIG. 20 shows an embodiment of a process flow for fabrication of solarmodules without a tempered glass cover;

FIGS. 21 and 22 show cross-sectional views of a solar glass assembly forbuilding façade applications;

FIG. 23 shows a view of an electrically conductive layer formed on aglass plate to interconnect cells in series for solar glassapplications;

FIG. 24 shows an embodiment of a process flow for fabrication of solarmodules for solar glass applications;

FIG. 25 serves as a reference FIGURE for calculation of TFSCinterconnect ohmic losses;

FIGS. 26 through 31 show graphs of interconnect (emitter contactmetallization) ohmic losses at maximum cell power versus the ratio ofemitter contact metal coverage height for various emitter metal sheetresistance values.

FIG. 32 outlines an embodiment of a process flow for fabrication of atemplate using photolithography patterning;

FIG. 33 shows a top view of an embodiment of a lithography mask designto produce a hexagonal array (honeycomb) pattern;

FIGS. 34 through 37 outline various embodiments of process flows forfabrication of a template using either direct laser micromachining orphotolithography patterning;

FIG. 38 shows the Y-Y and Z-Z cross-sectional axes on an embodiment of ahexagonal-prism (honeycomb) 3-D TFSC substrate;

FIGS. 39 and 40 show Y-Y cross-sectional views of an embodiment of atemplate including through-wafer and within-wafer trenches,respectively;

FIGS. 41 through 47 show Y-Y cross-sectional views of a siliconsubstrate during the fabrication process flow for making an embodimentof a template based on the process flows of FIG. 36 or FIG. 37;

FIGS. 48 through 52 show Y-Y cross-sectional views of alternativeembodiments of templates;

FIGS. 53 and 54 show embodiments of mask designs for patterning asemiconductor (silicon) wafer rear to produce backside openings on atemplate;

FIG. 55 shows an alternative frontside lithography mask with an array ofhexagonal array openings for formation of template trenches and an arrayof holes for formation of an array of release channels from the templatebackside to the template frontside;

FIG. 56 shows the frontside patterning mask in FIG. 55 with a backsidesquare array pattern (to be used for backside patterning with relativealignment as shown to the frontside pattern) superimposed for reference;

FIG. 57 shows the backside lithography mask pattern (square array) inFIG. 56 with the frontside mask hexagonal array pattern from FIG. 55superimposed for reference;

FIGS. 58 through 66 show Y-Y cross-sectional views of a semiconductor(silicon) substrate during the fabrication process flow for making anembodiment of a template based on the process flows of FIG. 36 or FIG.37;

FIGS. 67 through 75 show Y-Y cross-sectional views of a siliconsubstrate during the fabrication process flow for making an embodimentof a template based on the process flows of FIG. 36 or FIG. 37;

FIG. 76 and FIGS. 79 through 86 show Y-Y cross-sectional views of asemiconductor (e.g., silicon) substrate during the fabrication processflow for making an embodiment of a template based on the process flowsof FIG. 36 or FIG. 37;

FIGS. 77 and 78 show backside lithography mask designs; FIG. 78 showsthe relative alignment of the backside square array pattern with respectto the frontside hexagonal array pattern whereas FIG. 77 shows thebackside square array pattern used for formation of chemical releasechannels on the template.

FIGS. 87 and 88 show cross-sectional views of stacked templatestructures for concurrently fabricating and releasing twohexagonal-prism 3-D TFSC substrates per process pass (FIGS. 87 and 88show the stacked templates within-wafer trenches and through-wafertrenches, respectively);

FIGS. 89A through 95C show examples of several embodiments of 3-Dpolygon-prism TFSC substrates including various prism unit cellgeometrical designs and arrangements;

FIG. 96 shows an embodiment of a process flow for fabrication ofself-supporting hexagonal prism 3-D TFSC substrates using layer releaseprocessing;

FIGS. 97 through 102 show alternative embodiments of process flows forfabrication of self-supporting hexagonal-prism (as well as other prismarray patterns) 3-D TFSC substrates without rear base layers (to formdual-aperture 3-D TFSC substrates; i.e., 3-D TFSC substrates with topand bottom unit cell openings);

FIGS. 103 through 107 show Y-Y cross-sectional views of the evolution ofone prism unit cell of a template with through-wafer trenches, as itgoes through several key process steps for fabricating a hexagonal-prism3-D TFSC substrate (dual-aperture 3-D TFSC substrate) without a rearbase layer;

FIGS. 108 through 111 illustrate Y-Y cross-sectional views of anembodiment of a template with in-wafer trenches and no dielectric layerson the template frontside or template backside, as it goes throughseveral key process steps for fabricating a hexagonal-prism 3-D TFSCsubstrate (dual-aperture 3-D TFSC substrate) without a rear base layer;

FIGS. 112 through 115 illustrate Y-Y cross-sectional views of anembodiment of a template with through-wafer trenches and no dielectricson the template frontside, as it goes through several key process stepsfor fabricating a hexagonal-prism 3-D TFSC substrate (dual-aperture 3-DTFSC substrate) without a rear base layer;

FIGS. 116 and 117 show alternative embodiments of a process flows forfabrication of self-supporting hexagonal prism 3-D TFSC substratesincluding rear base layers (single-aperture 3-D TFSC substrates withsingle-aperture unit cells);

FIGS. 118 through 122 illustrate Y-Y cross-sectional views of a templatewith in-wafer trenches and no dielectrics on the template frontside, asit goes through the key process steps to fabricate a hexagonal prism 3-DTFSC substrate (single-aperture 3-D TFSC substrate) with a rear baselayer; FIGS. 121 and 122 show the released 3-D TFSC substrate with abase layer and the reusable template after the 3-D TFSC substraterelease, respectively.

FIGS. 123 through 125 illustrate Y-Y cross-sectional views of thetemplate in FIG. 66 with the rear-to-front release channels, as it goesthrough the key process steps to fabricate a hexagonal-prism 3-D TFSCsubstrate (single-aperture 3-D TFSC substrate) with a rear base layer(template is made on <100> silicon substrate);

FIGS. 126 through 128 illustrate Y-Y cross-sectional views of thetemplate in FIG. 75 with the rear-to-front release channels, as it goesthrough the key process steps to fabricate a hexagonal-prism 3-D TFSCsubstrate (single-aperture TFSC substrate) with a rear base layer(template is made on <110> silicon substrate);

FIGS. 129 through 131 illustrate Y-Y cross-sectional views of thetemplate in FIG. 86 with backside release channels aligned to the bottomof hexagonal-prism trenches, as it goes through the key process steps tofabricate a hexagonal-prism 3-D TFSC substrate with a rear base layer(single-aperture 3-D TFSC substrate);

FIG. 132 shows a 3-D cross-sectional view of an embodiment of asingle-aperture hexagonal-prism 3-D TFSC substrate (i.e., 3-D TFSCsubstrate with an integral base layer), including the substrate rearmonolithically (integrally) connected to a substantially flat planarthin semiconductor film;

FIG. 133 shows a top view of an embodiment of a regular (equilateral)hexagonal-prism 3-D TFSC substrate;

FIG. 134A shows a Y-Y cross-sectional view of an embodiment of a singleaperture hexagonal prism 3-D TFSC substrate, while FIG. 134B shows a Z-Zcross-sectional view;

FIGS. 135 through 140 show alternative process flow embodiments forfabricating hexagonal-prism 3-D TFSCs using single-aperture 3-D TFSCsubstrates including rear base layers;

FIG. 141 shows multiple adjacent hexagonal-prism unit cells, aftercompletion of the TFSC fabrication process and after mounting the cellrear base side onto a rear mirror;

FIGS. 142A through 144A show Y-Y cross-sectional views of a unit cellwithin an embodiment of a single-aperture hexagonal-prism 3-D TFSCsubstrate including a rear base layer;

FIGS. 144B through 148 show Y-Y cross-sectional views of an embodimentof a single-aperture hexagonal-prism 3-D TFSC substrate including a rearbase layer, and including either a detached or an integrated rearmirror;

FIG. 149 shows a schematic view of a double-sided coater setup forself-aligned application (coating) of dopant liquid or paste layers on3-D TFSC substrate hexagonal-prism top ridges and hexagonal-prism rearsurface or ridges by roller coating and in-line curing of the appliedliquid/paste layers (shown in conjunction with an integrated belt-drivenprocess equipment);

FIG. 150 shows a view of an alternative spray coater and curing setup toperform the same processes as the roller coater and curing setup of FIG.149;

FIG. 151 shows a view of another alternative setup design usingliquid-dip coating or liquid-transfer coating to perform the sameprocesses as the roller coater and curing setup of FIG. 149 and thespray coater and curing setup of FIG. 150;

FIG. 152 shows a 3-D view of an embodiment of a hexagonal-prism 3-Dthin-film semiconductor substrate after release and removal from atemplate;

FIG. 153A shows a schematic Y-Y cross-sectional view of an embodiment ofa dual-aperture hexagonal-prism 3-D TFSC substrate, while FIG. 153Bshows a Z-Z cross-sectional view of the same substrate;

FIGS. 154 through 159 show alternative process flow embodiments forfabricating hexagonal-prism 3-D TFSCs using dual-aperture 3-D TFSCsubstrates without rear base layers;

FIG. 160A shows a schematic Y-Y cross-sectional view of an embodiment ofa self-supporting (free-standing) hexagonal-prism dual-aperture 3-D TFSCsubstrate (without a base layer) including a thin peripheralsemiconductor (silicon) frame, before 3-D TFSC fabrication;

FIG. 160B shows a schematic Y-Y cross-sectional view of the 3-D TFSCsubstrate of FIG. 160A after TFSC fabrication;

FIG. 161A shows a schematic Y-Y cross-sectional view of an embodiment ofa self-supporting (free-standing) hexagonal-prism dual-aperture 3-D TFSCsubstrate including a thick peripheral semiconductor (silicon) frame,before TFSC fabrication;

FIG. 161B shows a schematic Y-Y cross-sectional view of the 3-D TFSCsubstrate of FIG. 161A after cell fabrication;

FIGS. 162A through 163B show schematic Y-Y cross-sectional views of asingle unit cell from a dual-aperture 3-D TFSC substrate within anembodiment of a hexagonal-prism 3-D TFSC fabricated using a 3-D TFSCsubstrate without a rear base layer;

FIGS. 164A and 164B show Y-Y cross-sectional views of a single unit cellfrom a dual-aperture 3-D TFSC substrate after mounting the cell onto arear mirror;

FIGS. 165A through 166 show Y-Y cross-sectional views of multiple unitcells from a dual-aperture 3-D TFSC substrate, after mounting onto arear mirror (with and without a spacing between the mirror and the rearcell);

FIGS. 167A through 171 show schematic Y-Y cross-sectional views of anembodiment of a hexagonal-prism 3-D TFSC formed on a dual-aperture 3-DTFSC substrate without a rear base layer, with substantially verticalhexagonal-prism sidewalls;

FIGS. 172A and 172B show 3-D views of a single unit cell in adual-aperture hexagonal-prism 3-D TFSC substrate, before and afterself-aligned base and emitter contact metallization, respectively;

FIG. 173 shows multiple adjacent hexagonal-prism unit cells, aftercompletion of the TFSC fabrication process and after mounting the cellrear base side onto a rear mirror;

FIG. 174 shows a schematic view of a single unit cell from an embodimentof a hexagonal-prism 3-D TFSC substrate for reference including certain3-D TFSC substrate calculations;

FIG. 175 shows a graph of the computed 3-D TFSC substratehexagonal-prism area ratio (ratio of 3-D cell surface area to the flatcell base area) versus hexagonal-prism aspect ratio (unit cell height toaperture diameter ratio);

FIG. 176 shows a graph of the ratio of the hexagonal-prism 3-D TFSCsubstrate mass to a reference flat semiconductor wafer mass for bothtypes of 3-D honeycomb-prism TFSC substrates (single and dual aperturesubstrates), versus various ratio of the honeycomb-prism sidewallsilicon thickness to the reference flat silicon wafer thickness;

FIG. 177 shows a schematic diagram of ray tracing for solar raysincident on a dual-aperture hexagonal-prism unit cell employingreflective emitter metallization contact;

FIGS. 178 through 181 show various numbers of solar light rays incidentat various angles of incidence, demonstrating efficient light trappingcharacteristics of the current disclosure;

FIG. 182 shows simulated light trapping in a unit cell and short circuitcurrent density versus angle of incidence for various emitter contactmetallization embodiments of the solar cell designs of the currentdisclosure;

FIG. 183 shows Standard Test Condition (STC) cell efficiency andshort-circuit current density for the solar cell of the currentdisclosure versus unit cell prism height;

FIG. 184 shows maximum photocurrent density versus incident angle, alsoindicating the effect of emitter contact metallization (assuming 100%optical reflectance for emitter contact metal);

FIG. 185 shows a graph of the representative selective emitterphosphorus and 3-D TFSC substrate boron doping profiles inhexagonal-prism 3-D TFSCs of this disclosure, shown with graded borondoping profile to create a built-in electric field;

FIG. 186 serves as a reference FIGURE for calculation of thehexagonal-prism TFSC internal ohmic losses due to the base current alongthe hexagonal-prism vertical sidewalls;

FIG. 187 shows maximum base resistivity and approximate p-type basedoping concentration values for various 3-D honeycomb-prism sidewallfilm thicknesses in order to limit the base current ohmic losses to lessthan 0.1%; and

FIG. 188 shows various views of silicon frames and silicon frame sliversfor the hexagonal-prism TFSCs of the current invention.

DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENTS

Preferred embodiments of the present disclosure are illustrated in thedrawings, like numbers being used to refer to like and correspondingparts of the various drawings. The innovative solar cell designs andtechnologies of the current disclosure are based on the use of athree-dimensional (3-D), self-supporting, doped (in one embodiment,in-situ-doped) semiconductor thin film, deposited on and released from areusable crystalline (embodiments include monocrystalline ormulticrystalline silicon) semiconductor template.

A preferred semiconductor material for the 3-D TFSC substrate iscrystalline silicon (c-Si), although other semiconductor materials mayalso be used. One embodiment uses monocrystalline silicon as the thinfilm semiconductor material. Other embodiments use multicrystallinesilicon, polycrystalline silicon, microcrystalline silicon, amorphoussilicon, porous silicon, and/or a combination thereof. The designs hereare also applicable to other semiconductor materials such as germanium,silicon germanium, silicon carbide, a crystalline compoundsemiconductor, or a combination thereof. Additional applications includecopper indium gallium selenide (CIGS) and cadmium telluridesemiconductor thin films.

The 3-D TFSC designs and production technologies as well as associatedmodule structures and assembly approaches of this disclosure effectivelyovercome the above-mentioned problems and challenges and enablecost-reduced fabrication of very-high-efficiency solar cells and modulesusing self-aligned cell process flows without the use of anyphotolithography patterning or screen printing or shadow-mask depositionprocess steps during cell fabrication (i.e., during 3-D TFSC substrateand cell fabrication after fabrication of the reusable 3-D template).The 3-D TFSC technologies of this disclosure are based on the formationof a 3-D prism-array thin-film TFSC substrate structure on a low-costreusable template and its subsequent release and lift-off from thetemplate to form a free-standing, self-supporting 3-D thin-filmsemiconductor substrate structure.

The current disclosure combines the benefits of TFSC fabrication on aproven high-efficiency crystalline silicon (c-Si) platform. The 3-D c-SiTFSC designs and technologies of this disclosure enable significantadvancements in the areas of c-Si solar cell and module efficiencyenhancement as well as manufacturing cost reduction. Based on innovativethin-film process steps, dependence on an expensive and constrainedsilicon wafer supply-chain is eliminated. Some of the unique advantagesof the cells designs and technologies of this disclosure which enableachieving ultra-high-efficiency at reduced manufacturing cost aresubstantial decoupling from the traditional solar PV silicon supplychain, performance enhancement, cost reduction, and reliabilityimprovement.

The disclosed subject matter improves solar cell efficiency by using a3-D c-Si film as an absorber layer in conjunction with highly efficientlight trapping. Use of the crystalline silicon absorber layer leveragesknown solar cell manufacturing techniques and supply chain, whilereducing absorber layer thickness (e.g., reduced by a factor of ten ormore compared to silicon wafers used for wafer-based solar cells). Thedisclosed method and system eliminates or substantially reducesphoto-degradation and enhances open-circuit voltage (V_(oc)) of cells.In addition, the disclosed method and system provides efficientfrontside and rear side light-trapping in conjunction with a highlyreflective rear mirror for maximum absorption of incident solar flux.Also, the disclosed method and system provides a selective emitter toenhance blue response and external quantum efficiency, with minimalshadowing of the cell and reduced ohmic losses due to a unique foldedemitter metallization contact design and improved module assembly.

Manufacturing cost is reduced by decreasing silicon usage (by asignificant factor, e.g., 3× to over 10×), with thinner deposited c-Sifilms also reducing the finished solar module energy payback time toless than 1 to 2 years. Manufacturing cost is further reduced byeliminating wire sawing and related kerf losses associated withmainstream solar cell wafer manufacturing technology. Manufacturing costis still further reduced by using self-aligned processing without anylithography or patterning steps used during the substrate and cellfabrication process flow, and a reduced number of fabrication processsteps, with improved yield and cycle time. Production cost is stillfurther reduced by using a simplified interconnection and cell-moduleassembly process and lightweight monolithic modules.

Operational reliability is improved by using thinner silicon films,eliminating photo-degradation and reducing temperature coefficients.Operational reliability is further improved by using a simpledistributed high-conductance electrical interconnection, minimizingfield failures. Operational reliability is still further improved byeliminating module glass cover (for glassless module assembly), thusreducing cost and facilitating field installation and operation.Operational reliability is still further improved by reducing the numberof manufacturing process steps and process variations using in-linemanufacturing process control.

The current disclosure reduces the solar module cost per watt for theuser (by at least 30% to 50%) and cuts balance-of-system (BOS) andinstallation costs for the integrators and installers. This may offermajor benefits to the global grid-tied end-users and solar systeminstallers and integrators. The current disclosure reduces the moduleintegration and installation cost and installed solar cell system costper W_(p) for the user, thereby lowering finished system cost per W_(p).The current disclosure increases module efficiency, with higher moduleefficiency resulting in lower BOS cost. The lower installed solar cellsystem cost results in reduction of the economic break-even time to alower fraction of the system lifetime, from roughly ½ to ⅓ for currentbest-of-breed c-Si solar cell systems to less than ¼ to ⅛ for theembodiments of this disclosure. The current disclosure reduces energypay-back time (EPBT) from 3 to 7 years for best-of-breed c-Si solar cellsystems to less than 1 to 2 years for the embodiments of thisdisclosure. Reduced EPBT substantially increases the net lifetime energyoutput (in kWh) for field-installed modules. The cell designs and moduleassemblies of this disclosure also provide stable degradation-free fieldoperation over an extended time (e.g., 30 to 40 year life of themodule), further increasing the net lifetime electrical energy output.Module manufacturing costs are expected to be 30% to 65% lower than thatof the leading high-performance c-Si solar cells/modules at the time ofmarket entry. This may shorten the ROI break-even time for the userscompared to the current industry roadmap and projections. Furtherbenefits include increased field performance stability and reliabilityand reduced environmental impact (non-toxic materials and shortenedEPBT). Further, the cell and module designs of this disclosure are idealfor grid-tied applications where it is advantageous to maximizeelectricity generation from a limited building rooftop or façade area.

The absorber silicon film thickness of the current disclosure may be avalue in the range of roughly 1 to 30 microns, where a thinner siliconlayer is preferred for less material consumption (in one embodiment, inthe range of 1 to 10 microns). Even after taking into account theeffective surface area increase due to the 3-D geometric structure ofthe 3-D TFSC substrates, the 3-D TFSC substrates of this disclosureconsume substantially less silicon material than the state-of-the-artwafer-based c-Si solar cells. Moreover, there are no sawing or kerflosses. Similarly, there is no requirement for saw damage removal sincethe 3-D crystalline silicon film is process-ready upon release from thereusable template. This substantially reduces the solar cell costassociated with silicon consumption. The self-supporting 3-D epitaxialsilicon thin film is deposited on and released from a low-cost reusablecrystalline (monocrystalline or multicrystalline) silicon substrate(template). The template may be reused numerous times before beingreconditioned or recycled. The template may even be chosen from the muchlower cost metallurgical-grade c-Si since any metallic impurities areprevented from contaminating the 3-D crystalline silicon film.

FIG. 3 summarizes the overall crystalline solar cell fabrication processflow of prior art techniques and highlights the specific stepseliminated by the current disclosure, compared to the prior art. FIG. 4summarizes the overall cell and module fabrication process flow and thecompetitive advantages of the current disclosure, compared to the priorart. As highlighted here, the current disclosure enables fabrication of3-D thin-film solar TFSC substrates and cells, thus, substantiallyreducing consumption of semiconductor absorber material (e.g., silicon)and the cell and module manufacturing costs. FIG. 5 provides anothersummary of the benefits of the embodiments of the current disclosure.

In the next section, various embodiments of this disclosure for makingsolar modules suitable for building rooftops and façades, centralizedpower generation, and other applications are described. Usually solarmodules are made by arranging a plurality of solar cells and connectingthem in series (series electrical connections) within a solar moduleassembly protected by a top glass layer and a rear protective materiallayer such as Tedlar. The cells may be connected in series in order tostep up the DC voltage (while maintaining the solar module current atthe same level at the level of the cell current) to facilitatehigh-efficiency DC-to-AC power conversion.

FIG. 6 shows a view of a representative example of series connections ofTFSCs of this disclosure in a solar module assembly 100. This exampleshows 24 squared-shaped cells 102 connected in series (in a 6×4 array).The electrical connections in series are shown by arrows between theadjacent cells connected in series. Module power input and output leads104 are also shown. In actual module assemblies, the numbers of cellsmay be smaller or larger and the cells may be connected in series or ina combination of series and parallel. As mentioned earlier, seriesconnection of the cells within the module assembly allows for steppingup the DC voltage for the DC-to-AC inverter (and also limiting the DCcurrent of the solar modules for ease of module installation in thefield and reliability of the module-to-module electrical connections).The printed-circuit-board (PCB) based module assembly of this disclosuresupports any number of cells assembled in a module and any electricalconnection configuration (series, series/parallel combination, orparallel). The TFSCs and modules of this disclosure may providerelatively lightweight solar modules with areas from less than 1 m² toseveral m² (e.g., 10 m²) for various applications. The cells connectedin series within a module assembly are chosen based on sorting to bematched in terms of their photogenerated current (e.g., short-circuitcurrent I_(sc) and/or maximum-power current I_(m)).

FIG. 7 shows a top view 110 of a hexagonal-prism 3-D TFSC with aperipheral planar silicon frame 112 for use in the above solar moduleassembly 100, shown in FIG. 6. The top surface of the frame 112 may alsobe used as the top 3-D TFSC interconnect and may be used to produce awrap-through or wrap-around emitter metallization for making contacts tothe cell emitter at the bottom of the cell (in module assembly). Theframe 112 is metallized, along with the top hexagonal emitter contacts,and is electrically connected to the hexagonal emitter contacts. Theframe 112 may have the same thickness as the 3-D TFSC substrate or maybe much thicker. In one embodiment, frame width 114 is between 5 and 500microns. The hexagonal prism 3-D TFSC substrate is composed ofhexagonal-prism unit cells 116. In one embodiment, the width 118 of thesilicon film forming the sidewalls of the hexagonal prism unit cell ispreferably 2 to 30 microns, and more preferably 2 to 10 microns.Typically, there are thousands to millions of hexagonal-prism unit cells116 on a large-area 3-D TFSC. In one embodiment, frame length (S) 120ranges from 125 to over 200 millimeters (e.g., 210 mm×210 mm). Thehexagonal-prism 3-D TFSC substrates of this disclosure may have a thinsilicon frame, a thick silicon frame, or no peripheral frame at all.

FIGS. 8A and 8B show microscopic views of 3-D thin-film TFSC substratesof a 3-D TFSC as illustrated in FIG. 7. FIG. 8A shows a view of adual-aperture TFSC substrate without a base layer whereas FIG. 8B showsa view of a single-aperture TFSC substrate with a base layer.

FIG. 9 provides an overview of the 3-D TFSC substrate and cellfabrication process flow. Focusing on the top of FIG. 9 illustrating the3-D TFSC substrate fabrication, note that the first step in this processflow uses a pre-fabricated template. The template with a pre-fabricated3-D trench or groove pattern may be used for formation of 3-D TFSCsubstrates, which are then used in the formation of 3-D TFSCs,substantially eliminating or reducing disadvantages and problemsassociated with previously developed TFSCs and the wafer-basedcrystalline silicon cell technologies. The template is capable of beingused numerous times (e.g., tens to hundreds of times) to fabricatenumerous 3-D TFSC substrates before being reconditioned or recycled. Inone embodiment, the template may be used hundreds of times to fabricate3-D TFSC substrates before being recycled. The template may be reusedfor as long as it remains relatively free of dislocations and/or for aslong as it maintains an acceptable trench or groove pattern with widthsand surface conditions within acceptable control limits (e.g., as gaugedby in-line metrology).

FIG. 10 shows a view 130 of a template with hexagonal-prism posts(pillars) 132. A hexagonal-prism 3-D thin-film TFSC substrate (notshown) is fabricated by first forming a suitable relatively conformalthin sacrificial layer (in one embodiment, porous silicon) on thetemplate, then filling in the relatively deep trenches 134 betweenhexagonal-prism posts 132, and subsequently releasing the hexagonalprism 3-D TFSC substrate by selectively etching the sacrificial layer(not shown) deposited between the hexagonal-prism 3-D TFSC substrate andthe template. In one embodiment, the template has deep interconnectedhexagonal-prism trenches with slightly tapered sidewalls (i.e., largertrench widths near the top of the trenched compared to near the bottomof the trenches. Moreover, the trench widths near the top of thetrenches may be made about one to several microns larger than the trenchwidths near the bottom of the trenches. FIGS. 11A and 11B show magnifiedviews of one embodiment of a template with hexagonal-prism posts 132 andtrenches 134. This embodiment was prepared using photolithography anddeep reactive-ion etching (DRIE).

Note that the terms “honeycomb” and “hexagonal” are used interchangeablythroughout this disclosure. The term “honeycomb” refers to the fact thatembodiments of the 3-D TFSC substrates resemble a natural honeycomb.

FIG. 12 shows a view 140 of an alternative embodiment of a template (ormaster stencil) with staggered square prism posts 142. A square-prism3-D TFSC substrate (not shown) is formed by first depositing or forminga relatively conformal sacrificial layer (e.g., porous silicon), fillingin the trenches 144 between square prism posts 142, and subsequentlyreleasing the 3-D TFSC substrate by selectively etching the sacrificiallayer formed between the 3-D TFSC substrate and the template.

In the next section, various embodiments of this disclosure for makingsolar modules suitable for building rooftops and façades, centralizedpower generation, and other applications are described. Usually solarmodules are made by arranging a plurality of solar cells and connectingthem in series (series electrical connections) within a solar moduleassembly protected by a top glass layer and a rear protective materiallayer such as Tedlar. The cells may be connected in series in order tostep up the DC voltage (while maintaining the solar module current atthe same level at the level of the cell current) to facilitatehigh-efficiency DC-to-AC power conversion.

The solar module structures and assembly methods of this disclosure arebased on the use of a printed-circuit board (PCB) to assemble thehexagonal prism 3-D TFSCs in a closely packed array and to connect thecells (in one embodiment in series) using the PCB plate within a moduleassembly. The PCB plate may have a single patterned metal (in oneembodiment, copper) interconnect layer on the top of the PCB or twopatterned copper layers on the top and rear surfaces of the PCB plate.FIG. 13 shows a view 150 of the frontside silver-coated copper layout ofthe printed-circuit board (PCB) used for solar module assembly (thesquare islands serve both as rear mirrors (if no integrated mirror isused with single-aperture cells, or if the cells are dual-aperture cellswithout base layers) and base interconnects; the peripheralsquare-shaped copper bands connect to the wrap-around emitter contact atthe TFSC peripheral frame rear side; copper-filled via plugs connectingselect regions of the PCB frontside and backside are shown as smallcircles). This example is shown for an array of 24 TFSCs arranged in 4rows of 6 cells in each row (the PCB may be designed for any number andvarious arrangements of TFSCs). The PCB conductor (copper or aluminum)thickness may be in the range of roughly 10 to over 100 microns toprovide high electrical and thermal conductivities. The PCB also servesas an effective heat sink to minimize temperature cycling of the TFSCsin operation. The PCB material may be selected to be a lightweight,high-strength material (such as carbon composite materials used inaerospace industry), or even a relatively thin flexible material. Thelarger-area square-shaped silver-coated copper regions 152 are connectedto the TFSC rear base regions (bottoms of the rear base layers for thesingle-aperture cells or the bottom ridges of the dual-aperture cellsfor the dual-aperture cells). The peripheral silver-coated copper lines154 are electrically connected to the TFSC emitter contact metallizationregions.

FIG. 14 shows a top view 160 of the backside (optionally silver-coated)copper layout of the printed-circuit board (PCB) used for solar moduleassembly, showing the series connection of the TFSCs. The PCB backsidemay also include thin-film shunt diodes for shade protection of theTFSCs (as shown in FIG. 15). The copper-filled via plugs (shown ascircles) connect the PCB frontside and backside metallization patternsin the corresponding areas. While the example shown here is forconnecting 24 TFSCs in series on a solar panel, similar PCB designmethodology may be applied to configure and connect any number of cellsin any desired arrangements on the module. The frontside view of thisPCB is shown in FIG. 13. This example is shown for an array of 24 TFSCsarranged in 4 rows of 6 cells in each row (the PCB may be designed forany number and various arrangements of TFSCs), all connected in series.The PCB conductor (copper or aluminum) thickness may be in the range ofroughly 10 to over 100 microns to provide high electrical and thermalconductivities. The PCB also serves as an effective heat sink tominimize temperature cycling of the TFSCs in operation. The PCB materialmay be selected to be a lightweight, high-strength material (such assuitable carbon composite materials used in aerospace industry). FIG. 14also shows power Output Lead 162 (first cell's p-lead) and power outputlead 164 (last cell's n-lead).

FIG. 15 shows a backside view 170 of the copper pattern on the PCB andis essentially similar to FIG. 14. This picture also shows the use ofprotective thin-film shunt diodes mounted on the PCB backside pattern(for cell shadow protection).

FIG. 16A shows an enlarged top view 180 of the silver-coated copperpattern (the pad for mounting one cell) on the frontside of the solarmodule printed-circuit board (PCB) used for rear mirror and also emitterand base interconnects for one of the TFSCs (relative dimensions are notshown to scale). FIG. 16A shows dimensions of L₁ 182 and L₂ 184 (in oneembodiment, 150 millimeters to greater than 200 millimeters, whereL₂=L₁+2(W+S)). S 186 may be on the order of 25 to 250 microns. The widthof the peripheral copper conductor band (W) 188 may be on the order of50 to 500 microns. The copper-filled via plugs 190 are shown as circles(connecting the interconnect patterns on the PCB frontside and backsidein a pre-designed arrangement in order to connect the TFSCs in series orin any other desired arrangement such as series/parallel; therepresentative example shown here is for connecting all the cells inseries in order to step up the module open-circuit voltage). The viaplug 190 diameters may be on the order of roughly 50 to 500 microns (andmay be smaller than W 188). The large central square pad serves both asthe rear cell mirror (for dual-aperture cells or single-aperture cellswithout integrated rear mirrors) and also base interconnect plane(connecting to the hexagonal-prism base contact metallization). Thenumber of vias in the center square (p-region contact) (N) 192 may be onthe order of hundreds to thousands. The number of vias in the peripheralline (n-region contact) (M) 194 may be on the order of tens to hundreds(or even thousands). The vias on the peripheral line contacting the TFSCemitter (n) regions are placed on three sides. The PCB conductor (copperor aluminum) thickness may be in the range of roughly 10 to over 100microns to provide high electrical and thermal conductivities. The PCBplate also serves as an effective heat sink to minimize temperaturecycling of the TFSCs in field operation. This FIGURE shows one of thecopper interconnect/mirror pads shown in the full module PCB array ofFIG. 13.

FIG. 16B shows an enlarged top view 200 of the silver-coated copperinterconnect pattern on the backside of the solar module printed-circuitboard (PCB) used for emitter and base electrical interconnects for acouple of adjacent TFSCs of this disclosure (a portion of the PCB view).FIG. 16B shows the PCB backside silver-coated copper interconnectpattern for TFSCs 1 and 2 in the array. The copper pattern here is shownfor connecting the TFSCs in series to step up the module open-circuitvoltage. FIG. 16B shows dimensions of L₁′ 202; peripheral emitter(n-region) connector linewidth W′ 204 (in one embodiment, 2 to 10millimeters); spacing between the center base (p-region) connector plateand the peripheral emitter (n-region) connector line S′ 206 (in oneembodiment, 100 microns to 1 millimeter). Note that L₁′ 202 is less thanL₁ from FIG. 16A by roughly 2 to 10 millimeters. This enables largerperipheral emitter (n-region) connector linewidth and substantiallyreduced ohmic losses on the PCB backside.

The PCB assembly described above may be used to create the final solarmodule assembly in a number of ways (with or without a frame, with orwithout top tempered glass, etc.).

FIG. 17 shows a cross-sectional view 210 of a solar module (solar panel)structure with a protective back plate 212 made of a proven prior artmaterial (e.g., Tedlar or polyvinyl fluoride film); a rear encapsulantlayer 214 (EVA), a 2-sided printed-circuit board (PCB) 216 of thisdisclosure with rear patterned electrical interconnects 218 and toppatterned electrical interconnects 220; cell rear mirrors (if applicablefor instance, for single-aperture cells with integrated rear mirrors)and TFSCs 222 with rear base and wrap-around (or wrap-through) emittercontacts mounted on the frontside of the PCB, a top encapsulate layer(EVA) 224, and an anti-reflection-coated (ARC) tempered glass (in oneembodiment, textured tempered glass) 226 (from rear to top), withgreater than 98% transmission, with sputtered or sprayed orliquid-coated anti-reflection coating). This module structure may beassembled as a hermetically sealed package either as a frameless moduleor with a frame (e.g., made of aluminum). In one embodiment, the moduleassembly is a frameless assembly (also for reduced materials energycontent and reduced energy payback time).

FIG. 18 illustrates a first embodiment of a process flow 230 forfabrication of solar modules with top protective glass plates andembedded PCBs of this disclosure (corresponding to the solar modulestructure of FIG. 17 with a PCB and a TFSC mounted on the PCB). Thismanufacturing flow is compatible with a fully automated module assemblyline. This module assembly flow is based on the use of a double-sidedprinted-circuit board (PCB) with the cell rear mirrors/baseinterconnects on the PCB topside (silver-coated patterned copper on thePCB topside). For hexagonal-prism 3-D TFSCs with rear base layers andintegrated/embedded (or attached) rear mirrors fabricated prior tomodule assembly (e.g., hexagonal-prism cells with rear base layers andthin-film rear mirrors deposited on the rear surfaces of the rear baselayers using PVD or plating or roller coating/spray coating and curing),the patterned PCB copper layer does not have to be coated with ahigh-reflectivity mirror material (silver). In step 232, module assemblystarts with a double-sided PCB coated with copper foils on bothfrontside and backside. The PCB area should support the desirednumber/layout of TFSCs (e.g., ≧1 m², with a copper foil thickness oneach side of roughly 10 to over 100 microns). Step 234 involves PCBinterconnect patterning and silver flash coating (the latter if neededfor PCB rear mirror). The PCB frontside and backside copper foils arepatterned according to the desired frontside and backside interconnectlayouts. Copper patterns are flash coated with a thin layer of highlyreflective silver (and/or aluminum). A highly reflective diffuse mirrormay be used, though a specular mirror may also be used. Step 236involves cell preparation for automated TFSC placement and soldering.The rear hexagonal metallized side of the TFSCs is roller coated (orspray coated or dip coated) with lead-free solder or an electricallyconductive and thermally-conductive epoxy paste. For cells fabricatedusing a honeycomb-prism TFSC substrate without a rear flat silicon baselayer, the rear metallized hexagonal-prism ridges are coated to avertical height of roughly 2 to 20 microns depending on the hexagonalprism unit cell dimensions. For single-aperture cells fabricated usinghoneycomb prism TFSC substrates with a rear flat silicon base layer,only the hexagonal base interconnect ridges are coated. Forsingle-aperture cells fabricated using honeycomb prism TFSC substrateswith a rear flat silicon base layer and an integrated rear base mirror,the coating may cover the entire rear base mirror bottom surface ifdesired. Step 238 involves automated TFSC placement and soldering (orcuring of epoxy). TFSCs are automatically picked and placed in aclosely-packed array on the frontside of the PCB. The rear side of eachcell sits on its designated site on the frontside of the double-sidedPCB with patterned copper interconnects. The TFSC rear hexagonal prismbase interconnect is soldered to the PCB frontside silver-coatedpatterned copper islands using thermal or ultrasonic soldering. In caseof using epoxy instead of solder, the epoxy layer is cured using thermaland/or IR/UV curing. The protective thin-film shunt diodes are mountedand soldered (or epoxied) on the PCB backside. An optional step is toflash coat the metal regions with a thin layer of highly reflectivesilver. Step 240 involves final solar module assembly and lamination. Astack of low-reflection tempered (in one embodiment, also textured) topglass, an encapsulant layer, the cell-mounted PCB, another encapsulantlayer and a Tedlar or polyvinyl fluoride back sheet is prepared. Next,the module stack assembly is hermetically sealed and packaged, forinstance, using vacuum-pressure lamination.

FIG. 19 shows a cross-sectional view 240 of another embodiment of asolar module structure. Instead of a top encapsulate layer (EVA) 224,and an anti-reflection-coated (ARC) tempered glass 226, as shown in FIG.17, there is a single frontside protective layer and anti-reflectivecoating layer 242. The frontside protective layer and anti-reflectivecoating (ARC) layer 242 is formed by liquid spray coating/curing, liquidroller coating/curing, liquid-dip coating/curing, plasma spray coating,or another suitable low-temperature coating technique. This frontsideprotective coating and ARC layer 242 is effectively textured for thecoating layer as deposited as a result of the 3-D structure of the TFSCs(thus, no separate texturing process is needed). This is due to the factthat the coating layer may have dips (low points) over the TFSChexagonal-prism cavities and peaks (high points) over thehexagonal-prism emitter ridges. The frontside protective layer andanti-reflective coating layer 242 may have a combined thickness in therange of tens to hundreds of microns. In one embodiment, the thicknessmay be approximately 30 to 300 microns. In addition to providing ananti-reflection coating (ARC) function, the stacked frontsideprotective/ARC layer provides excellent protection againstweather/elements and force impact (e.g., hail impact) in actual outdoorfield operation. Since the frontside coating is effectively andautomatically textured as a result of the 3-D structure of the TFSCs,the use of a separate ARC layer on the frontside coating is optional.The textured coating may provide effective light trapping in thefrontside coating for effective coupling of a very high fraction (e.g.,greater than 95%) of the incident solar light intensity to the TFSCs.The frontside protective layers may also provide an optical waveguidingfunction to eliminate or reduce any reflection losses associated withthe top emitter contact metallization.

FIG. 20 outlines an alternative embodiment of an assembly process flow250 for fabrication of reduced cost and reduced weight (lightweight)solar modules (corresponding to the solar module structure of FIG. 19).This flow is compatible with a fully automated module assembly. Thisprocess flow shows the assembly process without the use of a thick glassplate (thus, further reducing the weight, cost, and energy payback timeof the solar modules of this disclosure) and without an EVA encapsulantlayer on the top of the cells. The module topside (the frontside ofassembled cells) is covered with a hard protective glass-type layer (ifdesired, also including a top ARC layer) with a combined thickness onthe order of tens to hundreds of microns. As deposited, this frontsideprotective layer is effectively textured as a result of the 3-Dstructure of the TFSCs. The top layer may be formed by a liquid coatingtechnique (e.g., spray coating, liquid-dip coating, or roller coating)following by a thermal or UV curing process. The thermal (or UV) curefor the liquid-spray-coated (or liquid-dip coated or roller coated)protective/AR layers may be performed as a single step together with thevacuum-pressure thermal lamination process. This embodiment results in alightweight module assembly with reduced materials consumption, reducedcost, and reduced energy payback time. Step 252 (providing PCB)corresponds to step 232 in FIG. 18; step 254 (PCB patterning and silverflash coat) corresponds to step 234; step 256 (cell preparation)corresponds to step 236; and step 258 (automated TFSC placement)corresponds to step 238. Step 260 involves solar module lamination. Astack of the cell-mounted PCB, an encapsulant layer, and a back sheet isprepared. Next, a suitable hermetic sealing/packaging process such asvacuum-pressure lamination is performed. Step 262 involves deposition ofthe solar module frontside protective coating (which may beautomatically textured as deposited and provides efficient lighttrapping for effective coupling to the TFSCs) layer and an optional ARClayer. The frontside of the solar panel is coated with a thin layer ofprotective material (e.g., a glass-type transparent material) and anoptional top anti-reflection coating (ARC) layer using a suitablecoating method. This coating (roughly tens to hundreds of microns) maybe performed using liquid spray coating, liquid roller coating,liquid-dip coating, plasma spray coating or another suitable method.Next, a thermal/UV curing process is performed.

The hexagonal-prism 3-D TFSCs of this disclosure (both thesingle-aperture and dual-aperture cells with and without the rear baselayers) are inherently bifacial. The hexagonal-prism 3-D TFSCs of thisdisclosure (particularly hexagonal-prism cells without rear base layers)are uniquely suited for aesthetically appealing solar glass modules withuniform controlled light transmissivity for building façadeapplications. The hexagonal-prism 3-D TFSCs of this disclosure (thedesigns without rear base layers and without rear mirrors) provide veryuniform partial sunlight transmissivity through the cells. The averagelevel of sunlight transmissivity may be set by adjusting the prism unitcell aspect ratio (higher prism aspect ratios reduce the averagesunlight transmissivity through the cells).

FIG. 21 shows the schematic cross-sectional view 270 of a solar glassassembly using the hexagonal-prism TFSCs of this disclosure for buildingfaçade (architectural solar glass) applications. This is an embodimentof solar module assembly of this disclosure wherein the semi-transparentversions of the hexagonal-prism TFSCs of this disclosure (primarily thedual-aperture cells without rear base layers and without rear mirrors)are used for partially transparent solar glass modules for buildingfaçade applications. This example shows the semi-transparenthexagonal-prism cells 272 (the version without the rear base layer andwithout the rear mirror such that it provides some level of sunlight ordiffuse daylight transparency through the cell from frontside oroutdoors through the cell backside, to allow a portion of the incidentsunlight/daylight through the cell) mounted within a dual-paneargon-filled (gas-filled) low-E glass assembly. The partiallytransparent TFSCs of this disclosure are closely packed and placed onthe lower glass plate 274 (the glass plate facing the building indoors)which is coated with an optically transparent (or semi-transparent)patterned cell interconnect layer 276 to connect the cells in the solarglass in electrical series. The patterned cell interconnect layer 276may be made of a transparent conductive oxide (TCO) such as Indium TinOxide (ITO), a thin semi-transparent layer of silver, or a combinationthereof. The top glass plate 278 shown here may face the façade outdoorswhile the lower glass plate 274 (the one with the patternedtransparent/semi-transparent interconnect 276 formed on its innersurface) may face the building indoors. There is a sealed argon-filledspace 280 between the glass plates 274 and 2008. Further, there is asealing/support window frame 282 shown. This design allows for a veryuniform level of partial light transparency through the dual-pane solarglass module, thus, providing an aesthetically appealing solar glassdesign for architectural solar glass applications. The level of partialtransparency may be set by the hexagonal-prism cell geometricalparameters such as the unit cell hexagonal aperture size and the unitcell aspect ratio H/d. The level of partial light transparency may beincreased by reducing the unit cell aspect ratio H/d and/or byincreasing the unit cell aperture diagonal dimension d (see FIGS. 132and 190). Conversely, the level of partial light transparency may belowered by increasing the unit cell aspect ratio H/d and/or bydecreasing the unit cell aperture diagonal dimension d. Moreover, it ispossible to use a partially transparent rear mirror layer (in oneembodiment, a thin silver layer formed on the glass plate to form adiffuse partial rear mirror) as part of the patterned cell interconnectformed on the glass plate holding the attached cells. The partiallyreflecting/partially transparent rear mirror increases the effectivecell conversion efficiency, while reducing the partial lighttransmissivity through the solar glass assembly.

FIG. 22 shows another view 290, which is an enlarged, alternative viewof a portion of the solar glass module assembly shown in FIG. 21 forbuilding façade applications. This FIGURE has a magnified view of aportion of the solar glass with the hexagonal-prism cells (thus, therelative dimensions of the hexagonal prism cell and the solar glass arenot shown to scale). FIG. 22 shows frontside TFSC hexagonal emitterinterconnects 292 and self-aligned backside hexagonal base contact 294.The distance 296 between the top glass plate 278 and bottom glass plate274 may be between 1 and 12 millimeters. The hexagonal-prism cellparameters may be designed to allow for a desired level of lighttransmission through the cell (e.g., roughly 10% to 90%). The level ofaverage light transmissivity can be controlled by the aspect ratio ofthe TFSCs.

FIG. 23 shows a view 300 of a representative patterned semi-transparentor transparent electrically conductive layer 276 used for connecting thehoney-comb-prism TFSCs placed within the solar glass assembly in series(such as a transparent conductive oxide—TCO including indium-tin-oxideor ITO layer, or a thin semi-transparent layer of silver, or acombination thereof; which may also include a partially transparent cellrear mirror made of a suitable material such as an ultrathinsemitransparent layer of silver) formed on a glass plate 302 such as thelower glass plate 274 of FIG. 21. This example shows connection of6×4=24 TFSCs in series within a solar glass module assembly. Of course,a similar patterning methodology may be used for connecting any numberof TFSCs in series, or in a combination of series/parallel configurationwithin the solar glass assembly. Series connection of all the cellswithin a solar glass module assembly is a preferred electricalconnection method (in order to step up the solar glass output voltage,while maintaining the solar glass module current at the TFSC currentlevel). This pattern also shows the output electrical leads 304 of thesolar module (solar glass) assembly. The solar glass power electricalleads 304 may be fed through the solar glass frame assembly via ajunction box for electrical connections to the adjacent solar glassmodules. Patterned IR mirror and cell interconnects 306 are visible totransparent light. The pattern of deposited thin film layer (or multiplelayer structure) is formed by sputtering and/or plating. Note that theclear spaces shown between island and lines are typically smaller thanthose shown (FIGURE not to scale).

FIG. 24 shows an alternative embodiment of a module assembly processflow 310 for solar glass applications. This embodiment outlinesfabrication of semi-transparent solar glass modules for building façadeapplications (corresponding to the solar module structures of FIGS. 21,22, and 23). This solar glass module assembly flow is compatible with afully automated solar glass module assembly. This flow shows theassembly process using a dual-pane low-E glass structure (other glassstructures may be employed as well). This embodiment results in alightweight solar glass module assembly with reduced materialsconsumption, reduced cost, and reduced module energy payback time. Instep 312, solar glass manufacturing starts with a first glass platewhich may serve as the indoors side of a low-E architectural glassassembly for building façade. The glass area may be in a range from lessthan one m² to several m² depending on the application. Step 314involves formation of (semi)-transparent cell interconnect pattern onfirst glass plate. The glass plate is cleaned, and a patterned layer ofoptically transparent or semi-transparent electrically conducting layeris deposited to serve as the cell electrical interconnect plane. Thepatterned interconnect layer may be made of a transparent conductiveoxide (TCO) such as Indium-Tin-Oxide (ITO), a thin semi-transparentlayer of silver, or a combination thereof. The patterned layer may beformed by physical-vapor deposition (PVD) through a shadow mask oranother suitable technique. Step 316 involves cell preparation forautomated TFSC placement and attachment. The rear hexagonal metallizedside of the TFSCs is roller coated with lead-free solder or anelectrically conductive and thermally-conductive epoxy paste/liquid. Forcells fabricated using a honeycomb-prism TFSC substrate without a rearflat silicon base layer, the rear metallized hexagonal-prism ridges arecoated to a vertical height of roughly 1 to 20 microns depending on thehexagonal prism unit cell and base metal contact coverage dimensions.This process coats the base hexagonal array interconnects and theemitter wrap-around/wrap-through interconnects in preparation for cellplacement and attachment. Step 318 involves automated TFSC placement andsoldering (or curing of epoxy). TFSCs are automatically picked andplaced in a closely-packed array on the glass plate surface with thepatterned (semi)-transparent interconnects. The rear base sides ofhoneycomb-prism cells are placed on the glass plate. The TFSC rearhexagonal prism base interconnect is soldered (attached) to thepatterned cell interconnect layer on glass using thermal or ultrasonicsoldering. In case of using epoxy instead of solder, the epoxy is curedusing thermal or UV curing. Step 320 involves automated solarglass/module assembly. In one embodiment, the solar glass moduleassembly is prepared in an atmospheric argon-filled automated assemblyambient by: mounting the glass plate with the attached cells onto asolar glass frame (e.g., a metallic frame such as aluminum frame with aperipheral seal); and attaching a glass plate (in one embodiment with anAR coating (ARC) layer) in parallel to and spaced apart (e.g., byroughly 1 to 30 millimeters) from the other glass plate (comprising thecells), to the solar glass frame such that the cells are confined withinan argon-filled cavity formed between the two glass plates sealed by themetallic frame. This forms the low-E solar glass assembly with the cellsconfined and protected within the argon-filled cavity. The solar glassmodule frame also provides the electrical lead feedthroughs which areattached to the leads from the patterned interconnect. Module frameperipheral seals maintain the argon-filled cavity and prevent gasleakage.

One important consideration in the TFSC and module interconnects is thetotal power loss associated with the electrical interconnects in theTFSCs and the solar module assembly. The hexagonal-prism 3-D c-Si TFSCand PCB-based module designs of this disclosure effectively address thisissue, resulting in very low interconnect ohmic losses in the cells andwithin the module. This feature (in conjunction with the highlyefficient packing of the TFSCs on the PCB-based solar module assembly)substantially narrows the efficiency gap between the TFSCs and the solarmodule assembly in the technology of this disclosure.

The next section relates to the basic calculations of the emittercontact metallization ohmic losses in the hexagonal-prism 3-D TFSCs ofthis disclosure. The calculations of ohmic losses for emitter contactmetallization are also applicable to the hexagonal base contactmetallization. However, since several embodiments of this disclosuremount the hexagonal prism 3-D TFSCs on patterned printed circuit boards(PCBs), the base contact metallization is electrically connected in aplanar format to a very high conductivity copper pad; this substantiallyreduces the base interconnect ohmic losses (compared to the emitterinterconnect ohmic losses). Therefore, in practical embodiments of thisdisclosure, the interconnect ohmic losses are dominated by the emittercontact metallization.

FIG. 25 may be used for reference with an approximate analyticalcalculation of the TFSC interconnect ohmic losses, assuming a circularsubstrate with hexagonal-prism array of unit cells base on the celldesign embodiments of this disclosure. Since the overall cellinterconnect ohmic losses are dominated by the top emitter contactmetallization, the ohmic power loss due to the hexagonal emitter contactmetallization is calculated as a function of cell current at maximumpower and emitter contact metal vertical height coverage ratio L/d(ratio of the height of emitter contact metal coverage on the prismsidewall to the prism unit cell long hexagonal diagonal dimension). Theanalytical calculations shown here were used to produce the plots shownin the following FIGURES (FIGS. 26-31). The calculations performed andtrends obtained for round substrates are also approximately applicableto square-shaped TFSC substrates.

For the following calculations: I₀ is the total cell current at peakpower; R_(thm) is the sheet resistance of top hexagonal-coverage emittercontact metal; C is the effective flat surface coverage of hexagonalemitter contact metal with vertical height L; R_(eff)=R_(thm)/C, whereR_(eff) is the effective flat surface sheet resistance of top contactmetal; A=(πa²)/4; and J₀=(4I₀)/(πa²).

Interconnect Ohmic Losses @ Max Cell Power:

P ₁≅(R _(eff) I ₀ ²)/(8π)

C=[(8√3)/3](L/d)

R _(eff)=(√⅜)(d/L)R _(thm)

P ₁≅(R _(thm) I ₀ ²)[√3/(64π)(d/L)≅8.62×10⁻³(R _(thm) I ₀ ²)(d/L)

FIGS. 26 through 31 show plots of the calculated hexagonal-prism 3-DTFSC interconnect (due to the dominant emitter contact metallization)ohmic losses versus L/d (ratio of the vertical coverage height of theemitter contact metal coverage on the prism sidewall to the longdiagonal dimension of the hexagonal aperture of the hexagonal-prism unitcell), for various values of emitter contact metal sheet resistance(R_(thm)). Assuming a cell efficiency of 20%, a 200 mm×200 mmsquare-shaped cell based on one of the embodiments of this disclosureproduces roughly 8 W of photogenerated power (AM1.5) and a cell currentof roughly 12 A. Thus, in order to limit the maximum emitter contactmetallization ohmic losses to roughly 1% of the peak photogeneratedpower of roughly 8 W, the ohmic power losses should be limited to 0.08W.

FIG. 26 shows the interconnect (emitter contact metallization) ohmiclosses at maximum cell power (200 W/m²) versus the ratio of emittercontact metal coverage height (coverage height of emitter contact metalon the prism unit cell sidewall) to hexagonal aperture diagonaldimension (L/d) for an emitter contact metal sheet resistance ofR_(thm)=0.002 Ω/square (assuming a silver bulk resistivity of roughly1.6 μΩ/square, this corresponds to an 8 microns thick silver layer usedas the emitter contact metallization layer). In this case, L/d of morethan 0.03 may meet the requirement of less than 1% interconnect ohmiclosses (power loss less than 0.08 W). Thus, for d=150 microns, L≧4.5microns may meet the negligible (<1%) interconnect power lossrequirement. Similarly for d=300 microns, L≧9 microns may meet the lessthan 1% interconnect loss requirement.

FIG. 27 shows interconnect (emitter contact metallization) ohmic lossesat maximum cell power (200 W/m²) versus the ratio of emitter contactmetal vertical coverage height (coverage height of emitter contact metalon the prism unit cell sidewall) to hexagonal aperture diagonaldimension (L/d) for an emitter contact metal sheet resistance ofR_(thm)=0.005 Ω/square (assuming a silver bulk resistivity of roughly1.6 μΩ/square, this corresponds to a 3.2 micron thick silver layer usedas the emitter contact metallization layer). In this case, L/d of morethan 0.07 may meet the requirement of less than 1% interconnect ohmiclosses (power loss less than 0.08 W). Thus, for d=150 microns, L≧10.5microns may meet the negligible (<1%) interconnect power lossrequirement. Similarly for d=300 microns, L≧21 microns may meet the lessthan 1% interconnect loss requirement.

FIG. 28 shows interconnect (emitter contact metallization) ohmic lossesat maximum cell power (200 W/m²) versus the ratio of emitter contactmetal coverage height (coverage height of emitter contact metal on theprism unit cell sidewall) to hexagonal aperture diagonal dimension (L/d)for an emitter contact metal sheet resistance of R_(thm)=0.0075 Ω/square(assuming a silver bulk resistivity of roughly 1.6 μΩ/square, thiscorresponds to a 2.1 micron thick silver layer used as the emittercontact metallization layer). In this case, L/d of more than 0.12 maymeet the requirement of less than 1% interconnect ohmic losses (powerloss less than 0.08 W). Thus, for d=150 microns, L≧18 microns may meetthe negligible (less than 1%) interconnect power loss requirement.Similarly for d=300 microns, L≧36 microns may meet the <1% interconnectloss requirement. If the interconnect ohmic loss limit is raised toroughly 2% of the photogenerated power (i.e., 0.02×8=0.16 W), L/d>0.06may meet the requirement of less than 2% interconnect ohmic losses(power loss less than 0.16 W). Thus, for d=150 microns, L≧9 microns maymeet this revised interconnect power loss requirement. Similarly ford=300 microns, L≧18 microns may meet the <2% interconnect lossrequirement.

FIG. 29 shows the emitter contact metal ohmic losses at maximum cellpower (200 W/m²) versus the ratio of emitter contact metal coverageheight (coverage height of emitter contact metal on the prism unit cellsidewall) to hexagonal aperture diagonal dimension (L/d) forR_(thm)=0.010 Ω/square (corresponding to a 1.6 micron thick silver layerused as the emitter contact metallization layer).

FIG. 30 shows the emitter contact metal ohmic losses at maximum cellpower (200 W/m²) versus the ratio of emitter contact metal coverageheight (coverage height of emitter contact metal on the prism unit cellsidewall) to hexagonal aperture diagonal dimension (L/d) forR_(thm)=0.015 Ω/square (corresponding to a 1.07 micron thick silverlayer used as the emitter contact metallization layer).

FIG. 31 shows the emitter contact metal ohmic losses at maximum cellpower (200 W/m²) versus the ratio of emitter contact metal coverageheight (coverage height of emitter contact metal on the prism unit cellsidewall) to hexagonal aperture diagonal dimension (L/d) forR_(thm)=0.020 Ω/square (corresponding to a 0.8 micron thick silver layerused as the emitter contact metallization layer).

As shown in FIGS. 26 through 31, as the emitter contact metal (e.g.,silver) sheet resistance is increased (or the emitter contact metalthickness is reduced), the vertical coverage of the emitter contactmetal over the prism sidewall should be increased (as a fraction of thehexagonal prism unit cell aperture diameter) in order to maintain theinterconnect ohmic losses below a pre-specified threshold value (e.g.,less than 1%). In practice, the desired emitter contact metallizationmay comprise silver with a thickness on the order of 3 to 12 micronsthick and with a vertical height coverage on the order of 5 to 20microns.

As noted in FIG. 9, the first step in manufacturing a solar moduleassembly is the fabrication of a template.

In the following section, alternative embodiments of process flows forfabricating templates using either lithography and etch techniques orlaser micromachining (or laser drilling) are described. The templatesare then used and reused numerous times to fabricate the 3-D TFSCsubstrates with single-aperture or dual-aperture configurations (eitherwith or without rear base silicon layers) for 3-D TFSC fabrication,which are then used in the formation of 3-D TFSCs, which are used inthin-film solar module assemblies of the disclosed subject matter.

Templates may be fabricated using electronic-grade silicon wafers,solar-grade silicon wafers, or lower-cost metallurgical-grade siliconwafers. Moreover, templates made of silicon can be fabricated eitherusing monocrystalline or multicrystalline silicon wafers. The startingtemplate wafer may either be a standard polished wafer (after saw damageremoval) or even a lower grade wafer immediately after wire sawing(without saw damage removal). The latter may further reduce the cost ofthe templates. The relatively low cost of each template is spread overnumerous 3-D thin-film TFSC substrates, resulting in much lower TFSCsubstrate and finished module costs compared to the standardstate-of-the-art (e.g., 200 microns thick) solar-grade monocrystallineand multicrystalline silicon wafers and associated modules.

For further explaining how a template is fabricated, FIG. 32 shows anembodiment of a process flow 330. The process begins with step 332,where an unpatterned monocrystalline silicon or multicrystallinesilicon, either square-shaped or round substrate (e.g., 200 mm×200 mmsquare or 200-mm round) is provided. The starting template wafer may bea wafer prepared by wire saw either with or without saw damage removal(the latter may further reduce the cost of template). The startingtemplate wafer may also be made of a lower purity (and lower cost)metallurgical-grade silicon. In one embodiment, the substrate is roughly200 to 800 microns thick. Optionally, step 332 includes performinggettering on a low-cost metallurgical-grade silicon and/or performing asurface texturing etch (e.g., using isotropic acid texturing by amixture of nitric acid and hydrofluoric acid, or using alkalinetexturing in KOH/IPA) to create an optional textured template surface.Step 334 uses photolithography patterning (in one embodiment, using alower cost contact or proximity aligner/patterning) to produce aprism-array mask pattern such as hexagonal-array pattern in photoresist(i.e., interconnected hexagonal openings in the photoresist layer). Theprocess sequence includes the formation of an oxide and/or nitride(optional) layer, photoresist coating (e.g., spin-on or spray coating)and pre-bake, photolithography exposure through a hexagonal-array mask,and photoresist development and post-bake. One embodiment includes ahard mask layer (SiO₂ and/or SiN_(x); for example, a thin thermallygrown oxide layer can be used as an optional hard mask) below thephotoresist (although the process may be performed without the use ofany hard mask layer by placing the photoresist coating directly onsilicon). When using a hard mask layer, the exposed portions of the hardmask layer are etched after photoresist patterning (thus, forminghexagonal openings). Such etching of the exposed hard mask layer may besimply performed using a wet etchant such as hydrofluoric acid for oxidehard mask. Step 336 involves formation of hexagonal prisms usinganisotropic plasma etch; where a high-rate deep reactive ion etch (DRIE)process forms a closely-packed array of deep (e.g., 100 to 400 microns)hexagonal-shaped trenches in silicon. The photoresist and/or oxideand/or nitride hard mask layer(s) are used for pattern transfer from thepatterned photoresist layer to silicon. In one embodiment, the deep RIE(DRIE) process parameters are set to produce near-vertical, slightlytapered hexagonal-prism trench sidewalls. In an alternative embodiment,the deep RIE (DRIE) process parameters are set to produce roughly oressentially vertical hexagonal-prism sidewalls. Note that the slightlytapered sidewalls are preferred over the essentially vertical sidewalls.Step 338 involves template surface preparation and cleaning. Thisprocess includes stripping the patterned photoresist layer from thesubstrate. The template substrate is then cleaned in a wet bench priorto subsequent thermal deposition processing to form the TFSC substrates.Such cleaning may involve DRIE-induced polymer removal (using a suitablewet etchant such as a mixture of sulfuric acid and hydrogen peroxide)followed by an isotropic silicon wet etch (such as in a mixture ofnitric acid and hydrofluoric acid) in order to isotropically remove athin layer (e.g., on the order of 10 to 500 nanometers) of silicon fromthe trench sidewalls and bottoms. This may remove any surface and buriedcontaminants, such as any surface and embedded metallic and/orpolymeric/organic contaminants introduced by the deep RIE (DRIE)process, from the sidewalls and bottoms of the DRIE-produced templatetrenches. Template processing may complete after a deionized (DI) waterrinse and drying. Optionally and if desired, the template wafer may alsogo through a standard pre-diffusion (or pre-thermal processing) wafercleaning process such as a so-called RCA wet clean prior to theabove-mentioned DI water rinsing and drying. Another optional surfacepreparation step (either performed instead of or after the wet isotropicsilicon etch process) includes performing a short thermal oxidation(e.g., to grow 5 to 100 nanometers of sacrificial silicon dioxide),followed by wet hydrofluoric acid (HF) oxide strip (to remove anyresidual contaminants from the patterned template). If no optional oxidegrowth/HF strip is used, an optional dilute HF etch may performed toremove the native oxide layer and to passivate the surface with hydrogen(forming Si—H bonds) in preparation for subsequent 3-D TFSC substratefabrication. After the completion of step 338, the resulting templatemay then be used and reused multiple times to fabricate 3-D (e.g.hexagonal-prism) thin-film TFSC substrates.

FIG. 33 shows a top view of a lithography exposure mask design 340 whichmay be used for fabrication of a template, as described in step 334 ofprocess flow 330 above. Dark regions 342 are an opaque coating such asCr on a transparent mask plate. Light regions 344 are areas where theopaque coating (e.g., Cr) has been etched to allow for exposure of aphotoresist layer. In one embodiment, the width of the hexagonal line(L_(M)) 346 on the mask plate is between 1 and 30 microns, and thediagonal distance between hexagonal prism points (d) 348 or thehexagonal-prism aperture diameter is between 50 and 500 microns.

An alternative embodiment of a process flow 350 for patterning of atemplate is outlined in FIG. 34, which uses direct laser micromachininginstead of photolithography and reactive-ion etch. Step 354 (providingan unpatterned substrate) corresponds to step 332 of FIG. 32. Step 354involves the use of programmable precision laser micromachining to formthe desired periodic array of deep trenches. This process may beperformed in a controlled atmospheric ambient based on either physicalablation or a combination of physical ablation and laser-assistedchemical etching. Step 356 (surface preparation and cleaning)corresponds to step 338 of FIG. 32. After the completion of step 356,the resulting template may then be used and reused to fabricate multiple3-D TFSC substrates.

Another alternative embodiment of a process flow 360 for patterning of atemplate is outlined in FIG. 35, which uses photolithography and etch toproduce through-wafer trenches. Step 362 (providing an unpatternedsubstrate) corresponds to step 352 in FIG. 34. Step 364 involves forminga silicon dioxide (SiO₂) layer and/or a silicon nitride (SiN_(x)) layeron both the frontside and backside of the substrate (this step isoptional and may not be used). In one embodiment, the SiO₂ layerthickness is between 100 and 1000 nanometers. The SiO₂ layer is formedby steam oxidation or LPCVD and may be followed by a layer of SiN_(x)formed by LPCVD or PECVD. In one embodiment, the SiN_(x) layer thicknessis between 100 and 1000 nanometers. These layers may be formed on bothsides of the silicon substrate (as shown in FIG. 34), or only on thesubstrate frontside or backside. Alternatively, only one layer (eitheroxide or nitride) may be used. Step 366 (patterning) corresponds to step334 in FIG. 32; and step 368 (etch) corresponds to step 336. Step 370involves formation of backside oxide/nitride openings for 3-D TFSCsubstrate release etching. Photoresist lithography patterning and plasmaetch (or wet etch) are used to form a regular array of openings (e.g., asquare grid or a line pattern) in oxide/nitride on the substratebackside. These openings may be used during subsequent 3-D TFSCsubstrate fabrication (for wet etchant access to sacrificial layer frombackside). Step 372 (surface preparation and cleaning) corresponds tostep 338 in FIG. 32 and may be modified such that the surfacepreparation and cleaning process does not remove the dielectric layersfrom the substrate backside. After step 372, the resulting template maythen be used to fabricate 3-D TFSC substrates.

Another alternative embodiment of a process flow 380 for fabrication ofa template is outlined in FIG. 36, which uses photolithography and etch,enabling fabrication of TFSC substrates with a rear base layer andgrooves for formation of self-aligned base contacts. Another alternativeembodiment of a process flow 400 for patterning of a template isoutlined in FIG. 37, which uses photolithography and etch, enablingfabrication of TFSC substrates with a rear base layer and grooves forformation of self-aligned base contacts. FIGS. 41 through 47 show theY-Y cross-sectional views of a silicon substrate during the fabricationprocess flow for making a template based on the process flows of FIG. 36or FIG. 37. It may be useful to refer to FIGS. 41 through 47 whilereviewing the process flow steps of FIGS. 36 and 37.

Referring to FIG. 36, step 382 (providing an unpatterned substrate)corresponds to step 362 in FIG. 35; step 384 (forming oxide and/ornitride layers) corresponds to step 364; step 386 (patterning)corresponds to step 366; and step 388 (etch) corresponds to step 368.Step 390 involves formation of self-aligned shallow trenches which arewider than deep trenches. The self-aligned wider shallow surfacetrenches are formed by a timed selective isotropic dielectric (hardmask) etch to form hard mask undercuts with known lateral dimensionunder photoresist, stripping patterned photoresist, and a timedanisotropic silicon RIE to form shallower/wider tapered trenches nearsurface. Step 392 (formation of backside openings) corresponds to step370 in FIG. 35; and step 394 (surface preparation and cleaning)corresponds to step 372. After step 394, the resulting template may thenbe used and reused to fabricate multiple 3-D TFSC substrates. It shouldbe noted that the self-aligned wider shallow trenches (which are widerthan the deep trenches) may also be formed as part of the same deep RIEprocess which forms the deep trenches (i.e., steps 388 and 390 can bemerged into a single deep RIE process in a DRIE process equipment),thus, eliminating the need for the above-mentioned timed selectiveisotropic dielectric hard mask etch to form hard mask undercuts underphotoresist (this modified approach may also eliminate the need for thefrontside hard mask (i.e., the patterned photoresist layer can be formeddirectly on the substrate) and further simplify the template fabricationprocess). This simplified process can be performed by using a DRIEprocess recipe which first forms the deep hexagonal-prism trenches andsubsequently forms the shallow wider trenches (or shoulders) over thedeep trenches by performing a less anisotropic (or more isotropic)silicon etch process which primarily affects the upper (topmost) portionof the deep hexagonal-prism trenches. Using this modified approach thesidewall profile of the wider shallow trenches may be slightly orheavily tapered (both are acceptable).

Referring to FIG. 37, step 402 (providing an unpatterned substrate)corresponds to step 382 in FIG. 36. Step 404 involves forming a SiO₂layer and/or a SiN_(x) layer on the frontside and optionally on thebackside of the substrate. In one embodiment, SiO₂ layer thickness isbetween 100 and 1000 nanometers. The SiO₂ layer is formed by steamoxidation or LPCVD followed by a layer of SiN_(x) formed by LPCVD orPECVD. In one embodiment, the SiN_(x) layer thickness is between 100 and1000 nanometers. The layers are formed either on front or both sides ofthe silicon substrate. Alternatively, only one layer (oxide or nitride)may be used. Alternatively, a SiO₂ layer only on the frontside and aSiN_(x) layer only on backside may be formed. Step 406 (patterning)corresponds to step 386 of FIG. 36; step 408 (etch) corresponds to step388; and step 410 (formation of shallower wider trenches) corresponds tostep 390. Again and essentially as described for FIG. 36, theself-aligned wider shallow trenches (which are wider than the deeptrenches) may also be formed as part of the same deep RIE process whichforms the deep trenches (i.e., steps 408 and 410 can be merged into asingle deep RIE process in a DRIE process equipment). Step 412 involvesformation of an array of openings on the wafer backside of sufficientdepth to connect to at least some portions of the rears (bottoms) of thedeep trenches. These openings provide access to at least a portion ofeach prism unit cell from the substrate backside. These holes are formedby laser drilling (or may be formed using backside lithography and wetor plasma etch) and may be used for 3-D thin-film TFSC substrate releaseetching (for etchant access to sacrificial layer such as for etching thesacrificial porous silicon layer). Step 414 (surface preparation andcleaning) corresponds to step 394 of FIG. 36. After step 414, theresulting template may then be used and reused to fabricate multiple 3-DTFSC substrates.

FIGS. 36 and 37 result in templates which enable subsequent fabricationof 3-D thin-film TFSC substrates with rear base layers (e.g., such asflat rear silicon base layers) and interconnected shallow grooves ortrenches for formation of self-aligned high-conductivity base contactmetallization. These 3-D thin-film TFSC substrates may be used forsubsequent fabrication of high-efficiency TFSCs with self-aligned baseand emitter contacts. The dual-width trenches (or deep trenches withshallower and wider trench shoulders stacked on their top) in thetemplate enable fabrication of self-aligned base metallization contactsbeside self-aligned emitter metallization contacts.

In order to better understand the following FIGURES, FIG. 38 is providedto show a top view of a hexagonal prism 3-D TFSC substrate. FIG. 38shows the reference imaginary Y-Y and Z-Z cross-sectional axes on ahexagonal-prism 3-D TFSC substrate.

FIG. 39 shows a Y-Y cross-sectional view of a template 420 withthrough-wafer trenches 422 (i.e., trenches formed through the substrateand stopped on backside dielectric). This template 420 may be used tofabricate numerous hexagonal-prism 3-D TFSC substrates, including thosewithout rear base layers (i.e., dual-aperture TFSC substrates).

The template 420 has dimensions of h (horizontal distance betweentrenches) 424, T_(st) (trench top width) 426, H (height of the trench)428, T_(sb) (trench rear width) 430, and 2θ 432 (where θ is the averagesidewall taper angle). Note that because these are through-wafertrenches 422, H 428 is essentially the same as the silicon thickness ofthe template substrate. Because the through-wafer trenches 422 produceisolated posts, backside dielectric layer 434 is used and should besufficiently thick and strong to provide sufficient mechanical support.Backside dielectric layer 434 may be a single dielectric layer such asoxide (or nitride) or a stack of two or more dielectric layers such asoxide/nitride. In one embodiment, backside dielectric layer 434 iscomposed of a layer of LPCVD Si₃N₄ on top of a layer of thin thermalSiO₂. The template 420 contains a frontside etch-stop layer 436 (tophard mask layer). In one embodiment, the top hard mask 436 is composedof a layer 436 of LPCVD Si₃N₄ on top of a layer 438 of thin thermalSiO₂. Alternatively, the top hard mask layer 436 may include a singlelayer instead of a 2-layer stack (e.g., Si₃N₄, SiC_(x), etc.).Alternatively, there may be no top hard mask layer (patternedphotoresist formed directly on silicon).

FIG. 40 shows a Y-Y cross-sectional view of a template 440 with in-wafertrenches 442. This template 440 may also be used to fabricate numeroushexagonal-prism 3-D thin-film TFSC substrates, including those withoutrear base layers (i.e., dual-aperture TFSC substrates). The trenches 442are confined within the wafer (within the template substrate) and do notpenetrate the entire wafer thickness, leaving remaining wafer thicknessR 444; note that for a given template substrate thickness, H 446 is lessthan H 428 in FIG. 39. Thus, the wafer itself provides sufficientmechanical support without a need for mechanical support from backsidedielectrics (thus, eliminating the need for backside dielectrics;backside dielectrics are optional here).

The trenches formed in the templates shown in FIGS. 39 and 40 may havevertical sidewalls or slightly tapered sidewalls (in one embodiment,producing deep trenches with gradually and slightly decreasing trenchwidth moving from the trench top towards the trench bottom). In oneembodiment, sidewall angles are in the range of 0° to 10° (preferably inthe range of 0° to 1°). Trenches with negative or re-entrant sidewallangles (i.e., trenches with increasing trench width moving from thetrench top towards the trench bottom) are not desirable and may causedifficulty with 3-D TFSC substrate release and, therefore, should beavoided.

Both template 420 (FIG. 39) and template 440 (FIG. 40) are made usingone of the template process flows outlined in FIGS. 34-36. Theseflowcharts describe the preferred process steps used for fabricating thetemplates used for subsequent fabrication of numerous 3-D thin-film TFSCsubstrates.

FIGS. 41 through 47 show one embodiment of a process flow and evolutionof a template structure for a template version with in-wafer trenches442 and design to enable formation of self-aligned base contacts duringvarious stages of the template process flows outlined in FIGS. 34-36.

FIG. 41 shows a Y-Y cross-sectional view 450 after formation of aphotoresist frontside pattern 452 on dielectric (oxide) hard mask(backside dielectrics 434 are optional and may not be used). FIG. 42shows a Y-Y cross-sectional view 460 after anisotropic plasma oxide etch(or isotropic wet oxide etch) through the photoresist frontside pattern452. FIG. 43 shows a Y-Y cross-sectional view 470 after formation ofdeep hexagonal-prism trenches using deep RIE (DRIE). FIG. 43 furthershows remaining wafer thickness R′ 472 and trench height H′ 474. FIG. 44shows a Y-Y cross-sectional view 480 after timed selective isotropichard mask etch (e.g., oxide etch using HF) to form controlled lateralundercuts 482 under patterned photoresist 452 with width W_(ox). FIG. 45shows a Y-Y cross-sectional view 490 after photoresist strip. Note thatthe top hard mask layer 436 remains and the photoresist layer has beenremoved. FIG. 46 shows a Y-Y cross-sectional view 500 after anisotropicsilicon etch to form wider shallow trenches with controlled height (L)502 on the top of the narrower and deeper trenches 442. FIG. 47 shows aY-Y cross-sectional view of a completed template 510 after isotropicoxide etch to strip the top hard mask layer 436 as shown in FIG. 46.While shown here, the backside dielectric layers may also be removed (ormay not be used at all). This template 510 may also be used to fabricatenumerous hexagonal prism 3-D TFSC substrates. As described before, thecombination of deep trenches and wider shallow trenches (top shoulders)may be formed using a single DRIE process sequence (anisotropic deeptrench RIE followed by a less anisotropic silicon etch to form the topshoulders), thus, eliminating the need for the top dielectric hard masklayer 436 and the associated process steps reflected in FIGS. 44 and 45.

The following FIGURES (FIGS. 48 to 51) illustrate several alternativeembodiments of completed templates.

FIG. 48 shows a Y-Y cross-sectional view of a template 520 with in-wafertrenches 442 without a dielectric top mask layer or a dielectric rearmask layer. FIG. 49 shows a Y-Y cross-sectional view of a template 530with in-wafer trenches 442 without a dielectric top mask layer or adielectric rear mask layer, compared to the embodiment shown in FIG. 48.This view also shows template backside holes 532 used to allow for 3-DTFSC substrate release etching. These backside holes 532 may befabricated using either lithography and etch, or laser micromachining ordrilling. FIG. 50 shows a Y-Y cross-sectional view of a template 540with through-wafer trenches 422 without a top hard mask layer 436 asshown in FIG. 39. FIG. 51 shows a Y-Y cross-sectional view of a template550 with through-wafer trenches 422 without a top hard mask layer 436,compared to FIG. 46. Note further that the through-wafer trenches 422 inFIG. 51 have wider trenches (top shoulders) with controlled height (L)502 on the top of the narrower and deeper hexagonal trenches, like thetrenches in FIG. 47. However, note that FIG. 51 shows through-wafertrenches 422, whereas FIG. 47 shows within-wafer trenches 442.

For templates with through-wafer trenches, mechanical support may beprovided by either using a backside dielectric stack of sufficientstrength (such as oxide, nitride, polysilicon, or a combination thereofas described before), or using a backside-bonded silicon wafer. FIG. 52shows a view of a template 560 with through-wafer trenches and withoutany frontside dielectrics, suitable for fabrication of hexagonal-prismsingle-aperture 3-D TFSC substrates with rear base layers. This template560 includes a mechanical support rear silicon wafer 562 bonded at abonded interface 564 (e.g., via a dielectric such as oxide or adielectric stack 566 such as oxide/nitride between the wafers). Themechanical support rear silicon wafer 562 provides wet etchant access tothe template trenches through holes 568, which may be created either bylaser drilling or reactive ion etching. This template 560 enablesfabrication of 3-D TFSC substrates with capability for formation ofself-aligned base and emitter contacts during subsequent hexagonal prism3-D TFSC substrate fabrication. In an alternative embodiment, mechanicalsupport rear silicon wafer 562 may instead be formed by a layer ofpolysilicon deposited by LPCVD over the backside dielectric (ordielectric stack) 566, thus, eliminating the need for wafer bonding.

FIGS. 53 and 54 show views 570 and 580, respectively, of two examples ofmask designs (out of many possible designs), the first one asquare-array mask and the second one a line-array mask, which may beused to pattern the template backside to produce backside openings for3-D TFSC substrate release etching. This patterning is performed onlyonce on each template.

FIG. 53 shows a square-array mask 570, where each square-array unit cell572 has a square-array unit cell width 574 and a square array unit cellspacing 576. In one embodiment, both of these are approximately 1 to 5microns (may be smaller or larger as well). FIG. 54 shows a line-arraymask 580, where the pattern shown is repeated over the entire mask as aperiodic array. In one embodiment, the line widths and spaces are all 1to 10 microns (may be smaller or larger as well). The pattern has apattern width 582, which in one embodiment is approximately 50 to 500microns. Other mask patterns (e.g., lines, circles, etc.) enablingetchant access to remove the sacrificial layer may be used instead ofsquare array or orthogonal line array. Alternatively, it is possible touse laser drilling or laser micromachining instead of lithography/etchto create the backside holes/openings for etchant access.

An alternative to the backside patterning outlined in FIGS. 53 and 54uses a frontside mask to enable release of single-aperturehexagonal-prism 3-D thin-film TFSC substrates with flat base layers byproviding etchant access pathways from the template frontside.

FIG. 55 shows an alternative frontside hexagonal-prism mask design 590with center holes 592, shown as white circles on the mask plate. In oneembodiment, center holes 592 are roughly 1 to 5 microns in diameter.Note that the hexagonal-prism array design is the same as in FIG. 33.Dark regions 342 are opaque coating (e.g., Cr) on the mask plate. Lightregions 344 and 592 are areas to be etched. In one embodiment, the widthof the line mask (L_(M)) 346 is between 1 and 30 microns, and thediagonal distance between hexagonal-prism points (d) 348 is between 50and 500 microns.

FIG. 56 shows a view 600 of the template frontside mask design 590 shownin FIG. 55, also shown with dotted squares 572 indicating a superimposedimage of one embodiment of the backside mask design (in order to see therelative alignment of the frontside mask and backside mask from thefrontside mask perspective).

FIG. 57 shows a top view of a template backside mask design 610, withthe superimposed image of the hexagonal array of the mask design 590from FIGS. 55 and 56 shown as gray hexagonal-array pattern in order tosee the relative alignment of the frontside mask and backside mask fromthe backside mask perspective).

FIGS. 58 through 66 show an alternative template version during variousstages of the template process flows outlined in FIGS. 36 and 37.

FIG. 58 shows a Y-Y cross-sectional view of an n-type (e.g.,phosphorus-doped) [100] silicon substrate 620 after formation of a tophard mask layer 436 and a backside hard mask layer 434 using thermaloxidation. Note that before oxidation, an optional surface texturing wetetch (such as using an acid texturing etch or an alkaline texturingetch) may be performed using a suitable etchant such as KOH in order totexture the silicon surface. FIG. 59 shows the substrate 620 in FIG. 58after backside lithography to form a patterned photoresist layer 624comprising an array of square-shaped openings 622 and after wet oranisotropic plasma etching of the backside hard mask layer 434 in theexposed areas. Note that in order to use anisotropic wet etch to formthe backside channels, the backside mask square pattern for thesubstrate 620 backside is properly aligned to produce [111] sidewalls,[110] directed edges, and directed ribs. FIG. 60 shows the substrate 620in FIG. 59 after anisotropic etching of template from backside using ananisotropic wet etchant (e.g., KOH or TMAH) to form an array of pyramids626 with square bases and after stripping photoresist layer 624 fromtemplate backside. Note the angle 628 of the pyramids 626. In oneembodiment, this angle is 35.26°. The backside lithography mask squarepattern is properly aligned to produce [111] plane sidewalls 630, [110]directed edges, and [211] directed ribs. FIG. 61 shows the substrate 620in FIG. 60 after frontside patterning and anisotropic oxide plasma etch(or isotropic oxide wet etch) through patterned resist 452 inpreparation for formation of honeycomb-prism trenches and concurrentlyforming frontside-etched small-diameter release trenches 632, andremoving backside oxide layer 434. In one embodiment, the diameter(D_(R)) of the release access trenches 632 is between 1 and 5 microns.FIG. 62 shows the substrate 620 in FIG. 61 after frontside deep siliconRIE. Note that frontside-etched small-diameter trenches 632 connect tothe backside release channels 626 through shallower cone-shaped trenches634 (in one embodiment, at the centers of the hexagonal-prism posts).FIG. 63 shows the substrate 620 in FIG. 62 after timed selectiveisotropic hard mask (SiO₂) etch to form controlled lateral undercuts 482under patterned photoresist. FIG. 64 shows the substrate 620 in FIG. 63after photoresist 452 strip, with oxide hard mask 436 remaining. FIG. 65shows the substrate 620 in FIG. 64 after anisotropic siliconreactive-ion etch to form wider trenches with controlled height (L) 502on top of the narrower and deeper hexagonal-prism within-wafer trenches442. FIG. 66 shows the substrate 620 in FIG. 65 after isotropic oxideetch to strip top oxide 436. After this step an optional timed siliconwet etch may be performed in HNA or TMAH to remove about 5 to 500nanometers of silicon to remove any DRIE-induced trench sidewall damageand/or polymeric/metallic contamination. At this point, the substrate620 may serve as a reusable template for formation of 3-D thin-film TFSCsubstrates. Again as described before, the combination of deep trenchesand wider shallow trenches (top shoulders) may be formed using a singleDRIE process sequence (anisotropic deep trench RIE followed by a lessanisotropic silicon etch to form the top shoulders), thus, eliminatingthe need for the top dielectric hard mask layer 436 and the associatedprocess steps reflected in FIGS. 64 and 65. This alternative processflow also eliminates the need for the oxide hard mask (thus, photoresistcan be applied directly on silicon for frontside and backside patterningsteps).

FIGS. 67 through 75 show a template version during various stages of thetemplate fabrication process flow outlined in FIGS. 36 and 37. FIGS. 67through 75 are substantially similar to FIGS. 58 through 66, except theinitial silicon substrate is an n-type [110] substrate 640, whichresults in backside release channels 642 in the shape of rectangulartrenches with vertical sidewalls, rather than pyramids. The resultingsubstrate 640 shown in FIG. 75 may serve as a reusable template forformation of 3-D thin-film TFSC substrates. Again as described before,the combination of deep trenches and wider shallow trenches (topshoulders) may be formed using a single DRIE process sequence(anisotropic deep trench RIE followed by a less anisotropic silicon etchto form the top shoulders), thus, eliminating the need for the topdielectric hard mask layer 436 and the associated process stepsreflected in FIGS. 73 and 74. This alternative process flow alsoeliminates the need for the oxide hard mask (thus, photoresist can beapplied directly on silicon for frontside and backside patterningsteps).

Another approach to implement the template release channels is to placethem on the template substrate backside such that they connect to thebottoms of the hexagonal-prism deep trenches (instead of tops of theposts or pillars as shown before). FIGS. 76 and 79 through 86 show atemplate version (with the release channels connected to the bottoms ofthe deep trenches) during various stages of the template process flowsoutlined in FIGS. 36 and 37. This embodiment uses a backside lithographymask design as shown in FIG. 77 (other types of backside mask patternsfor backside release channels are also possible). FIG. 78 shows thebackside lithography mask shown in FIG. 77, with the frontsidehexagonal-prism array mask pattern shown as a superimposed gray patternfor reference (to show the relative alignment of the frontside andbackside masks patterns).

FIG. 76 is substantially similar to FIG. 58 above, showing an initialn-type (e.g., phosphorus doped) [100] substrate 620. FIG. 79 issubstantially similar to FIG. 59 above, except the mask design alignsbackside release channels 644 with the bottoms of deep prism trenches442 to be formed. FIG. 80 shows the substrate 620 shown in FIG. 79 afteranisotropic wet etching (e.g., using anisotropic alkaline etching suchas KOH-based etching) of the template backside to form an array ofpyramids with square bases (note that the anisotropic etching may alsobe performed using anisotropic reactive ion etching and the backsideopenings may be circular or other shapes instead of square-shaped). FIG.81 shows the substrate 620 in FIG. 80 after frontside patterning andafter wet oxide etch through patterned resist in preparation forformation of deep trenches. This also removes the backside oxide layer434. FIG. 82 shows the substrate 620 in FIG. 81 after formation ofhexagonal-prism trenches 442 using deep RIE (DRIE). Note that thebottoms of prism trenches 442 essentially align with the backsiderelease channel holes 644. FIG. 83 shows the substrate 620 in FIG. 82after timed selective isotropic hard mask (in one embodiment SiO₂) wetetch to form controlled lateral undercuts 482 under patternedphotoresist. FIG. 84 shows the substrate 620 in FIG. 83 after topphotoresist 452 strip using a photoresist stripper. FIG. 85 shows thesubstrate 620 in FIG. 84 after anisotropic silicon etch (using the oxidelayer as a hard mask) to form wider trenches (top shoulders) withcontrolled height (L) 502 on top of the narrower and deeperhexagonal-prism within-wafer trenches 442. FIG. 86 shows the substrate620 in FIG. 85 after isotropic oxide etch to strip top oxide 436. Afterthis step an optional timed isotropic silicon wet etch may be performedin HNA or TMAH (or another suitable isotropic silicon wet etchant) toetch approximately 5 to 500 nanometers of silicon to remove anyDRIE-induced trench sidewall contaminants (such as metallic and/orpolymeric contaminants) and surface damage. At this point, the substrate620 may serve as a template for formation of 3-D thin-film TFSCsubstrates. Again as described before, the combination of deep trenchesand wider shallow trenches (top shoulders) may be formed using a singleDRIE process sequence (anisotropic deep trench RIE followed by a lessanisotropic or more isotropic silicon plasma etch to form the topshoulders), thus, eliminating the need for the top dielectric hard masklayer 436 and the associated process steps reflected in FIGS. 83 and 84.This alternative process flow also eliminates the need for the oxidehard mask (thus, photoresist can be applied directly on silicon forfrontside and backside patterning steps).

Various embodiments of the templates shown earlier may be used toproduce one hexagonal-prism (or other prism geometries) 3-D thin-filmTFSC substrate per process pass. It is also possible to fabricatetemplates which are capable of producing two hexagonal-prism 3-Dthin-film TFSC substrates concurrently per process pass (thus, doublingthe 3-D TFSC substrate fabrication throughput). FIGS. 87 and 88 showcross-sectional views of two such templates capable of doubling thehexagonal-prism 3-D thin-film TFSC substrate production throughput.

FIG. 87 shows a Y-Y cross-sectional view of a stacked template structure650 for fabricating two hexagonal prism 3-D thin-film TFSC substratesper process pass. FIG. 87 shows a template structure 650 with in-wafertrenches 442. Note the similarity to the template 440 in FIG. 40.Template structure 650 is made of two similar templates, a top template652 and a rear template 654, which are first fabricated based on one ofthe embodiments outlined before and then bonded together backside tobackside (e.g., using direct thermal bonding of the wafer backsides orthermal bonding of dielectric layers formed on the wafer backsides) at abackside interface 656. Note that the dielectric hard masks on thetemplate frontsides may not be present (they are optional for subsequentuse of the templates for TFSC substrate fabrication). FIG. 88 shows Y-Ycross-sectional view of an alternative stacked template structure 660for concurrently fabricating two hexagonal-prism 3-D TFSC substrates perprocess pass. FIG. 88 shows a template structure 660 with through-wafertrenches 422. Note the similarity to the template 420 in FIG. 39.Template structure 660 is made of two similar templates, a top template662 and a rear template 664, which are first fabricated based on one ofthe embodiments outlined before and then bonded together backside tobackside (e.g., either through direct bonding of the substrate backsidesor using thermal bonding of dielectric layers formed on the waferbacksides) at a backside interface 656. Note that the dielectric hardmasks on the template frontsides may not be present (they are optionalfor subsequent use of the templates for TFSC substrate fabrication).

While FIGS. 87 and 88 show representative stacked template structuressuitable for higher throughput fabrication of hexagonal-prismdual-aperture 3-D thin-film TFSC substrates without rear base layers, itis also possible to make stacked template structures for fabrication ofhexagonal prism 3-D thin-film TFSC substrates with rear base layers.This may be done by first fabricating the suitable individual templatesbased on one of the process flow embodiments shown in FIGS. 36 and 37,corresponding to the template structure shown in FIG. 40 (this one shownwith in-wafer trenches; it is also possible to fabricate templates withthrough-wafer trenches such as the structures shown in FIG. 51 or FIG.52). Assuming we use a pair of templates with the structure shown inFIG. 40 (or a template structure with wider and shallow trenches orshoulders stacked on top of the deep trenches), these templates are thenprocessed to create a series of large lateral/radial microchannels inconjunction with an array of holes/openings which communicate with therears of trenches. The two templates are then bonded togetherbackside-to-backside (e.g., by thermal bonding of the backside surfacestogether). The radial/lateral microchannels sandwiched between thebonded wafers extend all the way to the periphery of the stacked/bondedtemplates and provide easy access for the wet etchant to reach thesacrificial layer (e.g., porous silicon formed by anodic etching ofmonocrystalline or microcrystalline silicon layer) in each template inorder to selectively remove the sacrificial layer in each template andto release the embedded hexagonal-prism 3-D thin-film TFSC substratesfrom the top and rear templates in the stack (thus, concurrently formingtwo hexagonal-prism 3-D thin-film TFSC substrates per process pass). Themicrochannels on the template backsides may be formed before templatebonding by laser ablation or a combination of lithography and etch. Themicrochannels are sufficiently large to allow for easy movement of wetetchant and etch byproducts between the inner portions of the wafers inthe bonded stack and the peripheral openings of the microchannels in themiddle of the bonded stack.

While one embodiment of the 3-D TFSC substrate unit cell structure ofthis disclosure is a regular hexagonal-prism unit cell (with equilateralhexagonal cross sections or ridges), this disclosure also covers a widerange of other 3-D prism unit cell geometrical designs with variouspolygon prism unit cell aperture designs. The prism array may be auniform array of a single polygon prism unit cell or a hybrid (two ormore) of multiple polygon-prism unit cell designs.

FIGS. 89A through 95C show examples of several 3-D polygon-prismthin-film TFSC substrates with various unit cell prism geometricaldesigns and arrangements. FIG. 89A shows a top view 670 of a prismdesign with hexagonal unit cell angles not equal to one another or 120°.FIG. 89B shows a top view 672 of a prism design with equilateraltriangular prism unit cells. FIG. 90A shows a top view 674 of a prismdesign with non-equilateral triangular prism unit cells. FIG. 90B showsa top view 676 of a prism design with alternating equilateral triangularprism unit cells. FIG. 91A shows a top view 678 of a prism design withoffset parallelogram prism unit cells. FIG. 91B shows a top view 680 ofa prism design with parallelogram prism unit cells. FIG. 92A shows a topview 682 of a prism design with aligned square prism unit cells. FIG.92B shows a top view 684 of a prism design with shifted square prismunit cells. FIG. 93A shows a top view 686 of a prism design with alignedrectangular prism unit cells. FIG. 93B shows a top view 688 of a prismdesign with shifted rectangular prism unit cells. FIG. 94A shows a topview 690 of a prism design with trapezoidal prism unit cells. FIG. 94Bshows a top view 692 of a prism design with alternating trapezoidalprism unit cells. FIG. 95A shows a top view 694 of a prism design withhybrid pentagon-parallelogram prism unit cells. FIG. 95B shows a topview 696 of a prism design with hybrid hexagon-triangle prism unitcells. FIG. 95C shows a top view 698 of a prism design with hybridoctagon-square prism unit cells.

In addition to these alternative TFSC designs, many other polygon-prismas well as other non-polygon prism unit cell designs (e.g.,cylindrical-prism, elliptical-prism, etc.) are covered by thisdisclosure. In general, the 3-D TFSC substrates of this disclosure coverany arrays of one or more prism unit cells arranged to make alightweight, enlarged-surface-area thin-film TFSC substrate for solarcell fabrication. Typically, there are millions (or as few as thousands)of these prism unit cells forming a large-area (e.g., 210 mm×210 mm) 3-DTFSC substrate. In one embodiment, the 3-D TFSC substrate film thicknessis in the range of 1 to 30 microns, and preferably in the lower-endrange of 2 to 10 microns. This is substantially less (by a factor ofroughly 20× to 100×) than the current state-of-the-art silicon solarcell wafer thickness (roughly 200 microns).

As noted in FIG. 9, the templates described above may be used forformation of 3-D thin-film TFSC substrates, which are then used in theformation of 3-D TFSCs, which are used in thin-film solar moduleassemblies of the disclosed subject matter.

FIGS. 96 through 102 show various embodiments of hexagonal-prism 3-Dthin-film TFSC substrate fabrication process flows for fabricatinghexagonal-prism (or other prism-array shapes) dual-aperture 3-Dthin-film TFSC substrates without rear base layers based on the use oftemplates described earlier. These 3-D thin-film TFSC substrates arethen used in the formation of 3-D dual aperture TFSCs. The resultinghexagonal prism 3-D dual aperture TFSCs are uniquely suited foraesthetically appealing solar glass modules as shown in FIG. 21.

All of the embodiments shown in FIGS. 96 through 102 use sacrificiallayer formation (e.g., porous silicon sacrificial layer) and trench-filldeposition processes (e.g., epitaxial silicon deposition) which may behighly conformal, for conformal formation of the sacrificial (poroussilicon) layer and subsequent seamless void-free filling of the trencheswith a semiconductor absorber layer such as in-situ-doped (e.g., in-situboron doped) monocrystalline or multicrystalline silicon layer. Oneembodiment uses a patterned single-crystal (monocrystalline) silicon ormulticrystalline silicon (mc-Si) square-shaped (or round) template, withdimensions of approximately 150 mm×150 mm to over 200 mm×200 mm.Alternative embodiments may use much lower cost metallurgical-grade orsolar-grade silicon.

FIG. 96 shows an embodiment of a process flow 700 for fabrication ofself-supporting (free standing) hexagonal-prism 3-D thin-film TFSCsubstrates using layer release processing. This process flow results indual-aperture hexagonal-prism 3-D TFSC substrates with hexagonal prismswith open apertures formed on both the top and rear (there is no rearbase layer). In step 702, a patterned hexagonal-prism (or another prismarray) template is provided. This template has already been processed toform an embedded array of deep hexagonal-prism trenches. There is apatterned dielectric (oxide and/or nitride) hard mask on the templatetop and rear surfaces. Step 704 involves a multi-layer blanket epitaxialsemiconductor deposition in an epitaxial growth reactor. Step 704 firstinvolves an H₂ or GeH₄/H₂ in-situ bake cleaning, which is performedafter a standard pre-epitaxial wet clean (the latter if necessary).Next, a thin sacrificial epitaxial layer is deposited on the frontsideonly. In one embodiment, Ge_(x)Si_(1-x) is used for the sacrificialepitaxial layer and is between 10 and 2000 nanometers (in anotherembodiment a layer of porous silicon is directly deposited for thesacrificial layer). Next, a doped monocrystalline silicon epitaxiallayer is deposited (in one embodiment, on the frontside only). In oneembodiment, the layer is p-type, boron-doped and has a thickness between1 and 30 microns. Step 706 involves selective silicon etch toselectively strip the top silicon layer, stopping on the sacrificiallayer. First, the top silicon layer is removed using a selective (wet ordry) silicon etch process until the top Ge_(x)Si_(1-x) epitaxial layer(or porous silicon) or oxide/nitride hard mask is exposed. When using aplasma (dry) etch process, one embodiment uses optical end-pointdetection to ensure complete removal of the top silicon layer andexposure of the top sacrificial (Ge_(x)Si_(1-x) or porous silicon)layer. Step 708 involves 3-D thin-film TFSC substrate release using aselective etchant to etch the sacrificial layer. A highly selectiveisotropic (in one embodiment, wet) etch of Ge_(x)Si_(1-x) is performed,with very high selectivity with respect to silicon (in one embodiment,with etch selectivity much better than 100:1). In one embodiment, amixture of hydrofluoric acid, nitric acid and acetic acid (HNA) is usedto etch the sacrificial Ge_(x)Si_(1-x) layer (etchants such as H₂O₂+H₂Oor TMAH may be used to selectively etch porous silicon). Alternatively,a mixture of ammonia, peroxide, and water (NH₄OH+H₂O₂+H₂O) may be used.This process releases the crystalline silicon layer as a hexagonal-prism3-D thin-film TFSC substrate, which may then be used for subsequent 3-DTFSC fabrication.

FIG. 97 shows a process flow 710 of an embodiment of a process flow forfabrication of self-supporting hexagonal-prism dual-aperture 3-Dthin-film TFSC substrates using layer release processing. This processflow results in hexagonal-prism 3-D thin-film TFSC substrates comprisinghexagonal-prisms with open apertures formed on both the 3-D thin-filmTFSC substrate top and rear (there is no rear base layer). In step 712,a patterned hexagonal-prism template is provided. This template hasalready been processed to form an embedded array of deep trenches asdescribed before. In this case, there are no dielectric hard mask layerson the template top and rear surfaces. Step 714 (multi-layer blanketepitaxial deposition) corresponds to step 704 in FIG. 96; step 716(selective silicon etch) corresponds to step 706; step 718 (substraterelease) corresponds to step 708. The resulting hexagonal-prism 3-Dthin-film TFSC substrate may then be used for subsequent 3-D TFSCfabrication.

In the process flows described in FIGS. 96 and 97, while crystallineGe_(x)Si_(1-x) is used as a sacrificial layer between the template andthe in-situ-doped epitaxial silicon layer, other suitable sacrificiallayers may be used. Alternative suitable materials include crystallineporous silicon (embodiments include microporous or mesoporous silicon),crystalline GeSiC, or SiC, among others. In the embodiments which useGe_(x)Si_(1-x) as the sacrificial layer, the Ge_(x)Si_(1-x) layer ispreferably between 50 nanometers and 3 microns thick, and morepreferably between 0.1 and 1 micron. The germanium mole fraction (x) inthe sacrificial layer is preferably between x=10% to x=45%, and morepreferably between x=25% to x=40% in order to provide sufficient etchselectivity with respect to silicon. The germanium mole fraction shouldbe high enough to achieve an acceptable high etch selectivity forremoval of the Ge_(x)Si_(1-x) layer with minimal etching of silicon,while low enough to produce high-quality epitaxial silicon withsufficiently low defect density over the Ge_(x)Si_(1-x) sacrificiallayer. The sacrificial layer may have a fixed germanium mole fractionthroughout the sacrificial Ge_(x)Si_(1-x) layer or may also have gradedmole fraction transition regions between the Ge_(x)Si_(1-x) layer andthe template as well as between the Ge_(x)Si_(1-x) layer and the topin-situ-doped epitaxial silicon layer. Graded mole fraction transitionregions result in a trapezoidal germanium mole fraction profile, firstincreasing from an x of 0% to an x of between 25% and 40% in the firsttransition region next to the template, then a Ge_(x)Si_(1-x) regionwith a fixed x between 25% and 40%, and a second transition region withx decreasing from an x of 40% to an x of 0% under the in-situ-dopedepitaxial silicon layer. The transition regions with graded x and themiddle layer with a fixed x may each be roughly between 10 nanometersand 1 micron thick.

Depending on the emitter doping type (n-type or p-type), the in-situbase doping type is chosen to be p-type (e.g., boron) or n-type (e.g.,phosphorus). The embodiments shown in FIGS. 96 and 97 provide examplesof boron-doped hexagonal prism 3-D thin-film TFSC substrates which maybe used to fabricate TFSCs with n-type, phosphorus-doped selectiveemitters. In an alternative embodiment, all the doping polarities may beinverted, resulting in phosphorus-doped hexagonal-prism 3-D thin-filmTFSC substrates which may be used for fabricating cells with boron-dopedselective emitters.

FIGS. 96 through 98 show embodiments of process flows which utilizeGe_(x)Si_(1-x) as the sacrificial layer between the template and theepitaxial silicon TFSC substrate. FIGS. 96 and 97 depict embodiments ofprocess flows which utilize blanket epitaxial silicon deposition as partof the process flow. On the other hand, FIG. 98 shows an alternativeembodiment of a process flow 720 using selective epitaxial silicondeposition and layer release processing. The resulting 3-D dual-aperturethin-film TFSC substrates of FIG. 98 have open apertures on bothsubstrate top and rear. In step 722, a patterned hexagonal-prismtemplate is provided. This template has already been processed to forman embedded array of deep trenches. There is a patterned dielectricoxide and/or nitride (and/or another suitable dielectric such asdiamond-like carbon or DLC, thin-film diamond, etc.) hard mask on thetemplate top surfaces. Step 724 (multi-layer blanket epitaxy)corresponds to step 704 in FIG. 96. Note, however, that both theGe_(x)Si_(1-x) sacrificial layer and the epitaxial silicon layer areselectively grown inside the trenches only. No layer is grown on thetemplate top dielectric layer due to selective epitaxy (there is apatterned dielectric hard mask on top, such as oxide, nitride, DLC,etc.). Step 726 (substrate release) corresponds to step 718 in FIG. 97.The hexagonal-prism 3-D thin-film TFSC substrate may then be used forsubsequent 3-D TFSC fabrication.

FIG. 99 depicts a process flow 730 of an embodiment of a process flowfor fabrication of self-supporting hexagonal-prism 3-D dual-aperturethin-film TFSC substrates without rear base layers using layer releaseprocessing. Process flow 730 uses monocrystalline orquasi-monocrystalline porous (microporous or mesoporous) silicon(instead of Ge_(x)Si_(1-x)) as the sacrificial layer, and blanketepitaxial silicon fill. The dual-aperture hexagonal-prisms have openapertures on both top and rear. Step 732 (providing a substrate)corresponds to step 722 in FIG. 98. Step 734 involves forming a thinporous silicon sacrificial layer on template deep trenches (trenchsidewalls and bottoms) using electrochemical HF etching (also known aselectrochemical anodization of silicon). The porous silicon layer may beformed by one of two primary techniques as follows: (i) deposit a thinconformal crystalline silicon layer (in one embodiment, a p-typeboron-doped silicon layer in the range of 0.2 to 2 microns) on an n-typetemplate substrate, using silicon epitaxy, followed by conversion of thep-type epitaxial layer to porous silicon using electrochemical HFetching; or (ii) convert a thin layer of the template substrate (in oneembodiment, a p-type template) to porous silicon (in one embodiment, inthe thickness range of 0.01 to 1 micron). The sacrificial porous siliconformed by one of these two techniques also serves as a seed layer forsubsequent epitaxial silicon deposition. Step 736 involves performing ahydrogen bake (at 950° to 1150° C.) to clean the surface and to form acontinuous sealed monocrystalline surface layer on the surface of theporous silicon sacrificial layer, followed by depositing a blanket layerof doped silicon epitaxy (top only) in an epitaxial processing reactor.In one embodiment, the layer is p-type, boron-doped and has a thicknessbetween 1 and 30 microns. Step 738 uses a selective (wet or dry) siliconetch process to strip the top silicon layer, until the top layer ofporous silicon is exposed. When using a plasma (dry) etch process, oneembodiment uses optical end-pointing to ensure complete removal of topsilicon layer and exposure of the top porous silicon layer. Step 740involves 3-D thin-film TFSC substrate release. A highly selectiveisotropic wet or dry etch of porous silicon is performed, with very highselectivity with respect to silicon. In one embodiment, a mixture ofhydrofluoric acid, nitric acid and acetic acid (HNA) is used to etch theporous silicon layer. Alternatively, a mixture of ammonia, peroxide, andwater (NH₄OH+H₂O₂+H₂O) or a mixture of hydrogen peroxide andhydrofluoric acid (H₂O₂+HF) or a suitable composition oftri-methyl-ammonium-hydroxide (TMAH) may be used. The etch compositionand temperature may be adjusted to achieve maximum etch selectivity forporous silicon with respect to silicon. This process releases theembedded 3-D crystalline silicon structure as a hexagonal-prism 3-Dthin-film TFSC substrate, which may then be used for subsequent 3-D TFSCfabrication.

FIG. 100 depicts a process flow 750 of an embodiment of a process flowfor fabrication of self-supporting dual-aperture hexagonal-prism 3-Dthin-film TFSC substrates without rear base layers using a poroussilicon sacrificial layer and selective epitaxial silicon deposition.The main difference between FIGS. 99 and 100 is the epitaxial growthmethod. FIG. 100 uses selective epitaxy instead of blanket epitaxy as inFIG. 99. The use of selective epitaxy eliminates a process step to etchthe top silicon layer before removal of the sacrificial layer for layerrelease. Referring to FIG. 100, step 752 (providing a substrate)corresponds to step 732 in FIG. 99; and step 754 (formation of poroussilicon sacrificial layer) corresponds to step 734. In step 756,epitaxial silicon is selectively grown inside the template trenchesonly. In one embodiment, the epitaxial silicon layer is p-type,in-situ-boron-doped and has a thickness between 1 and 30 microns. Nosilicon layer is grown on the template top surface due to selectiveepitaxial growth, as there is a patterned dielectric (e.g., oxide and/ornitride) hard mask on top. Step 758 (substrate release) corresponds tostep 738 in FIG. 99. The hexagonal-prism 3-D thin-film TFSC substratemay then be used for subsequent 3-D TFSC fabrication. While FIGS. 99 and100 show the use of porous silicon sacrificial layers for fabrication ofdual-aperture TFSC substrates (without rear base layers), porous siliconcan also be used as a sacrificial layer for fabrication ofsingle-aperture TFSC substrates with rear base layers (using theappropriate template structure for single-aperture TFSC substrates, asdescribed before).

The preceding FIGURES outline process flow embodiments which result inhexagonal-prism 3-D TFSC substrates made of a crystalline(monocrystalline or multicrystalline) semiconductor material (preferablycrystalline silicon), through the use of conformal epitaxial depositiontechniques. In alternative embodiments, 3-D TFSC substrates are madefrom polycrystalline or amorphous semiconductor materials (such aspolysilicon or amorphous silicon). However, the resulting 3-D TFSCs mayexhibit lower efficiencies compared to the 3-D TFSCs made from acrystalline semiconductor (e.g., monocrystalline or multicrystallinesilicon).

FIGS. 101 and 102 depict embodiments of two process flows 760 and 780for fabrication of self-supporting hexagonal-prism 3-D dual-aperturethin-film TFSC substrates without rear base layers using layer releaseprocessing but without the use of silicon epitaxy, unlike FIGS. 96 to100 above. The template used in FIG. 101 has a patterned dielectric onthe template frontside, while the template used in FIG. 102 has nodielectrics on either the frontside or backside. The sacrificial layerused in these process flows is simply a sacrificial dielectric layersuch as SiO₂. The silicon material is amorphous silicon and/orpolysilicon, which may optionally be subsequently crystallized usinglaser crystallization to form large-grain polysilicon ormulticrystalline silicon (and thermally annealed in order to reducesilicon film stress to facilitate release from the template). Theseprocess flows result in 3-D dual-aperture thin-film TFSC substrates withhexagonal-prism unit cells with open apertures on both top and rear.

Referring to FIG. 101, step 762 (providing a substrate) corresponds tostep 752 from FIG. 100. Step 764 involves depositing a conformalsacrificial layer (or a layer stack). First, a thin layer of asacrificial material is deposited by conformal layer formation (LPCVD orthermal oxidation). In one embodiment, the sacrificial material is SiO₂,with a thickness of between 50 and 2000 nanometers. This sacrificialoxide layer conformally covers the hexagonal-prism trench walls and thetemplate frontside. If subsequent laser crystallization is used (seestep 772 below), step 764 also includes depositing a thin nitride layerby LPCVD. In one embodiment, this nitride layer is Si₃N₄, with athickness between 100 and 1000 nanometers. The sacrificial layer may bemade of porous silicon instead of oxide and/or nitride. Step 766involves deposition of a blanket silicon layer using conformaldeposition. In one embodiment, this blanket silicon layer may beamorphous silicon or polysilicon, p-type in-situ doped with boron,having a thickness between 1 and 30 microns. Step 768 uses selectivesilicon (wet or dry) etch to strip the top silicon layer, until the topsurface of sacrificial layer (e.g., silicon dioxide or silicon nitrideor porous silicon) is exposed. When using plasma (dry) etch process, oneembodiment uses optical end-pointing to ensure complete removal of topsilicon layer and exposure of the top porous silicon layer. If optionalnitride is used, the top Si₃N₄ layer is etched using selective wet ordry etch. Step 770 involves 3-D thin-film TFSC substrate release. In oneembodiment and when using a silicon dioxide sacrificial layer,hydrofluoric acid (HF) is used to etch the oxide sacrificial layer. Inanother embodiment and when using a porous silicon sacrificial layer, amixture of ammonia, peroxide, and water (NH₄OH+H₂O₂+H₂O) or a mixture ofhydrogen peroxide and hydrofluoric acid (H₂O₂+HF) or a suitablecomposition of tri-methyl-ammonium-hydroxide (TMAH) may be used. Theetch composition and temperature may be adjusted to achieve maximum etchselectivity for porous silicon with respect to silicon. This processreleases the hexagonal-prism 3-D thin-film TFSC substrate. An optionalstep 772 involves laser crystallization and/or thermal annealing of thereleased 3-D thin-film amorphous silicon or polysilicon substrate toform a large-grain polysilicon microstructure (and to reduce anyresidual stress for the embedded silicon structure), where the nitridelayer serves as protective cap. The nitride layer is then selectivelystripped. The hexagonal prism 3-D thin-film TFSC substrate may then beused for subsequent 3-D TFSC fabrication.

Referring to FIG. 102, in step 782, a patterned hexagonal-prism templateis provided. This template has already been processed to form anembedded array of deep interconnected trenches. In this case, there areno dielectric hard mask layers on the template top and rear surfaces.Step 784 (conformal deposition of sacrificial layer) corresponds to step764 of FIG. 101; step 786 (blanket silicon deposition) corresponds tostep 766; step 788 (selective silicon etch) corresponds to step 768;step 790 (substrate release) corresponds to step 770; and step 792(optional laser crystallization) corresponds to step 772. Thehexagonal-prism 3-D thin-film TFSC substrate may then be used forsubsequent 3-D TFSC fabrication.

FIGS. 103 through 107 show Y-Y cross-sectional views of the evolution ofone prism unit cell of a template with through-wafer trenches 422, as itgoes through several key process steps for fabricating a hexagonal-prismdual-aperture 3-D thin-film TFSC substrate without a rear base layer.The process flow outlined in these FIGURES includes the use of bothblanket and selective epitaxial growth based on the relevant processflow embodiments described earlier. While shown with a dielectric stackon the template frontside and backside, the process flow using blanketepitaxial deposition may proceed without using the dielectric stack onthe template frontside.

FIG. 103 shows a Y-Y cross-sectional view 800 of a template withthrough-wafer trenches 422 after deposition of a thin sacrificial layer(epitaxial Ge_(x)Si_(1-x) or porous silicon) 802 followed by depositionof a blanket in-situ-doped (e.g., boron-doped for p-type base) epitaxialsilicon layer 804. The dielectric top hard mask layer is composed of afirst top hard mask layer 436 of Si₃N₄ on top of a second top hard masklayer 438 of SiO₂ on top of the template top surface 806. Thesacrificial layer 802 forms a thin layer on both the sidewalls 808 andon the template top surface 806 (being formed on the top surface of thefrontside dielectric 436). This sacrificial layer 802 forms a thin layer(e.g., 100 to 2000 nanometers thick) between the in-situ-doped(boron-doped) epitaxial silicon 804 and the template.

FIG. 104 shows a view 810 after a controlled silicon etch to remove thep-type silicon epitaxial layer 804 from the top of the template only(leaving epitaxial silicon in trenches). FIG. 105 shows a template view820 after a highly selective wet etch process to remove the sacrificiallayer 802 (Ge_(x)Si_(1-x) or porous silicon or another suitable layer)shown in FIG. 104. Removal of the sacrificial layer 802 results in theformation of a gap 972 between the template and the p-type siliconepitaxial layer (i.e., the 3-D thin-film TFSC substrate) 802, allowingfor release and removal of the 3-D thin-film TFSC substrate from thetemplate. Release of the substrate 804 may be facilitated by ultrasonicagitation and/or magnetically coupled etchant stirring during selectivewet etching of the sacrificial layer 802. FIG. 106 shows a view 830 ofthe template after lifting off the p-type silicon epitaxial layer 804.The template 420 (see FIG. 39) is ready for multiple reuse cycles.

FIG. 107 shows three cross-sectional views. View 840 shows a larger Y-Ycross-sectional view of the template shown in FIG. 106, with multipleprism unit cells shown. View 842 shows a Y-Y cross-sectional view of thehexagonal-prism 3-D TFSC substrate 804 after release from the templateshown in view 840 (with the emitter side down). Note that the base andemitter sides are shown on the top and rear, respectively. View 844shows a Y-Y cross-sectional view of the hexagonal-prism 3-D thin-filmTFSC substrate 804 (vertically flipped view compared to view 842) afterrelease from the template shown in view 840. Note that the emitter andbase sides are shown on the top and rear, respectively.

FIGS. 108 through 111 show Y-Y cross-sectional views of the evolution ofone prism unit cell within a template with in-wafer trenches 442 and nodielectric layers on the template frontside or template backside, as itgoes through several key process steps for fabricating a hexagonal-prismdual-aperture 3-D TFSC substrate without a rear base layer. The processflow outlined in these FIGURES includes the use of both blanket andselective epitaxial growth based on the relevant process flowembodiments described earlier.

FIG. 108 shows a Y-Y cross-sectional view 850 of a version of thetemplate 520 (see FIG. 48) with in-wafer trenches 442 without adielectric top mask layer or a dielectric rear mask layer. The deeptrenches may be formed using lithography patterning and deep RIE (DRIE).FIG. 109 shows a view 860 after deposition of the relatively thin (e.g.,200 to 2000 nanometers) sacrificial layer 802 (epitaxial Ge_(x)Si_(1-x)or porous silicon or another suitable material) and the in-situ-doped(boron doped for p-type base or phosphorus doped for n-type base)epitaxial silicon layer 804 to fill the hexagonal-prism trenches. FIG.110 shows a view 870 after controlled plasma or wet etching of thein-situ-doped (boron-doped for p-type base) epitaxial silicon layer 804on the template frontside, endpointing or stopping the etch on the topof or within the frontside sacrificial layer 802. FIG. 111 shows a view880 after highly selective wet etching of sacrificial layer 802(epitaxial Ge_(x)Si_(1-x) or porous silicon) to release thehexagonal-prism 3-D thin-film TFSC substrate 804. The dual-aperture 3-DTFSC substrate with embedded silicon sidewalls 804 is ready to bedetached and removed from the template.

FIGS. 112 through 115 illustrate Y-Y cross-sectional views of a template540 (see FIG. 50) with through-wafer trenches 422 and a backside layer434 (trenches penetrating through the template bulk and terminated onthe supporting backside dielectric layer 434) and no dielectrics on thetemplate frontside, as it goes through the key process steps tofabricate a hexagonal-prism dual-aperture 3-D TFSC substrate without arear base layer. Again, the flow used for this fabrication process flowis based on one of the embodiments outlined earlier.

FIG. 112 shows a Y-Y cross-sectional view 890 of a version of thetemplate 540 with through-wafer trenches 422 without a top hard masklayer, but with a backside mechanical support layer 434. FIG. 113 showsa view 900 after formation or deposition of the relatively thinsacrificial layer 802 (epitaxial Ge_(x)Si_(1-x) or porous silicon oranother suitable material) and the in-situ-doped (boron-doped epitaxialfor p-type base) epitaxial silicon layer 804. FIG. 114 shows a view 910after controlled plasma or wet etching of the epitaxial silicon layer804 on the template frontside, endpointing or stopping the etch on thetop of or within the frontside sacrificial layer 802. FIG. 115 shows aview 920 after selective wet etching of the sacrificial layer 802(epitaxial Ge_(x)Si_(1-x) or porous silicon) to release thehexagonal-prism 3-D thin-film TFSC substrate 804. The dual-aperture 3-DTFSC substrate with embedded silicon sidewalls 804 is ready to bedetached and removed from the template.

The following section describes embodiments of hexagonal-prism 3-Dthin-film TFSC substrate fabrication process flows for fabricatinghexagonal-prism dual-aperture 3-D thin-film TFSC substrates with rearbase layers.

FIGS. 116 and 117 show two different process flow embodiments forfabricating hexagonal-prism dual-aperture 3-D thin-film TFSC substrateswith rear base layers (using a suitable template such as the one shownin FIG. 47). FIG. 116 depicts an embodiment of a process flow 930 usinglayer release processing. This flow is based on the use ofGe_(x)Si_(1-x) sacrificial layer deposition and blanket or selectivein-situ-doped epitaxial silicon deposition. The resultinghexagonal-prism unit cells have open apertures on prism top and areterminated at the rear with a rear base layer (in one embodiment, arelatively flat thin silicon layer). Again, the process flow of thisembodiment may be easily adjusted in order to use polysilicon, amorphoussilicon, or a non-silicon crystalline or polycrystalline/amorphoussilicon material. In step 932, a patterned honeycomb-prism template isprovided. This template has already been processed to form an embeddedarray of trenches along with shallower/wider trenches (or trenchshoulders) stacked on top of narrower/deeper trenches (see FIG. 47).There is no dielectric layer on the template frontside, and there is apatterned oxide and/or nitride dielectric layer (or stack) with openingsleft on the template backside. In step 934, a multi-layer blanketepitaxy is performed in an epitaxial reactor, including the followingin-situ process steps. First, H₂ bake or GeH₄/H₂ bake is used forin-situ surface cleaning. Next, a thin Ge_(x)Si_(1-x) epitaxial layer isdeposited (in one embodiment, on the top only). In one embodiment, thislayer is between 10 and 1000 nanometers. Next, a doped silicon epitaxiallayer is deposited on the top only. In one embodiment, this layer isp-type, boron-doped and between 1 and 30 microns thick. The in-situdoping (boron doping) profile may be flat or graded. In case of grading,boron doping concentration is gradually increased during the depositionof the silicon epitaxial layer, with a lower concentration at thebeginning and a higher concentration towards the end of the epitaxialgrowth process. This graded base doping may provide a field-assisteddrift component for efficient collection of photo-generated carriers,substantially reducing the impact of recombination losses. It alsoreduces base sheet resistance and ohmic losses. The silicon epitaxiallayer thickness is set such that the deep trenches are fully filled withsilicon while the shallow (wider) trenches (top trench shoulders)receive epitaxy on their sidewalls and their central regions are leftwith self-aligned shallow hexagonal troughs. In step 936, the 3-Dthin-film TFSC substrate is released. A highly selective isotropic wetor dry etch of Ge_(x)Si_(1-x) is performed, with very high selectivitywith respect to silicon. In one embodiment, a mixture of hydrofluoricacid, nitric acid and acetic acid (HNA) is used to selectively etch theGe_(x)Si_(1-x) layer. Alternatively, a mixture of ammonia, peroxide, andwater (NH₄OH+H₂O₂+H₂O) may be used. The wet etchant selectively removesthe sacrificial Ge_(x)Si_(1-x) layer by reaching the sacrificial layerthrough the template backside dielectric openings. This process releasesthe hexagonal prism 3-D TFSC substrate, which may then be used forsubsequent 3-D TFSC fabrication. Note that the template backsideopenings may be formed directly in silicon backside without a need forthe backside dielectric.

FIG. 117 depicts an embodiment of a process flow 940 for fabrication ofself-supporting hexagonal-prism single-aperture 3-D thin-filmpolysilicon or amorphous silicon TFSC substrates with rear base layersmade of polysilicon or amorphous silicon using layer release processing,without the use of epitaxial silicon processing. The amorphous siliconor polysilicon layer may be optionally crystallized using lasercrystallization as part of the flow. This process flow uses a dielectricsacrificial layer such as SiO₂ (deposited using LPCVD or thermallygrown) in conjunction with conformal amorphous silicon or polysilicondeposition for the silicon absorber layer. Step 942 (providing asubstrate) corresponds to step 932 in FIG. 116. Step 944 involvesdepositing a conformal sacrificial layer (or a layer stack). First, athin layer of a sacrificial material is deposited by conformal layerformation (LPCVD or thermal oxidation). In one embodiment, thesacrificial material is SiO₂, with a thickness of between 50 and 2000nanometers. This sacrificial oxide layer conformally covers thehexagonal-prism trench walls and the template frontside. If subsequentlaser crystallization is used (see step 952 below), step 944 alsoincludes depositing a thin nitride layer by LPCVD. In one embodiment,this nitride layer is Si₃N₄, with a thickness between 100 and 1000nanometers. The sacrificial layer may be made of porous silicon insteadof oxide and/or nitride. Step 946 involves deposition of a blanketsilicon layer using conformal deposition. In one embodiment, thisblanket silicon layer may be amorphous silicon or polysilicon, p-typein-situ doped with boron, having a thickness between 1 and 30 microns.Note that the silicon thickness is set such that the deep trenches arefully filled with silicon while the shallow (wider) near-surfacetrenches receive silicon on sidewalls, and their central regions areleft with self-aligned relatively shallow hexagonal troughs or trenches.Step 948 involves depositing an optional thin silicon nitride dielectriclayer on top by LPCVD or PECVD to serve as a protective cap for siliconlayer. In one embodiment, this layer is between 100 and 1000 nanometers.Step 950 involves 3-D thin-film TFSC substrate release. In oneembodiment and when using a silicon dioxide sacrificial layer,hydrofluoric acid (HF) is used to etch the oxide sacrificial layer. Inanother embodiment and when using a porous silicon sacrificial layer, amixture of ammonia, peroxide, and water (NH₄OH+H₂O₂+H₂O) or a mixture ofhydrogen peroxide and hydrofluoric acid (H₂O₂+HF) or a suitablecomposition of tri-methyl-ammonium-hydroxide (TMAH) may be used. Theetch composition and temperature may be adjusted to achieve maximum etchselectivity for porous silicon with respect to silicon. Note that thewet etchant selectively removes the sacrificial Ge_(x)Si_(1-x) layer (orporous silicon sacrificial layer) by reaching the sacrificial layerthrough the template backside dielectric openings (note that backsideopenings may be formed directly in the template substrate backsidewithout using any dielectric on the template backside). This processreleases the hexagonal-prism 3-D thin-film TFSC substrate from thetemplate. An optional step 952 involves laser crystallization of thereleased 3-D thin-film amorphous silicon or polysilicon substrate toform a large-grain polysilicon microstructure. The silicon nitride layersurrounding silicon serves as protective cap. The nitride layer is thenselectively stripped. The hexagonal-prism 3-D thin-film TFSC substratemay then be used for subsequent 3-D TFSC fabrication.

While multiple embodiments are shown in FIGS. 96 to 102 and FIGS. 116and 117 to depict various hexagonal-prism 3-D TFSC substrate fabricationmethods of this disclosure, the embodiments shown may be used to developadditional process flows and variations based on the overall concepts,designs, embodiments, and methodologies.

FIGS. 118 through 122 illustrate Y-Y cross-sectional views of a template510 (see FIG. 47) with in-wafer hexagonal-prism trenches 442 and nodielectrics on the template frontside and an optional backside layer 434(the template may be fabricated without any frontside and backsidedielectric layers), as it goes through the key process steps tofabricate a hexagonal-prism single-aperture 3-D thin-film TFSC substratewith a rear base layer. Again, the flow used for this fabricationprocess flow is based on one of the embodiments outlined earlier.

FIG. 118 shows a view 960 after deposition of the thin (e.g., 200 to2000 nanometers thick) sacrificial layer 802 (epitaxial Ge_(x)Si_(1-x)or porous silicon or another suitable material) and the in-situ-doped(boron-doped for p-type base) epitaxial silicon layer 804. The epitaxialsilicon deposition process fills the trenches (void-free trench fill)while leaving relatively shallow troughs (trenches) near the top. Thismay be done by stopping the epitaxial deposition process after thedeeper/narrower trenches are fully filled with epitaxial silicon andbefore filling of the wider/shallower trenches on the template frontside(thus, forming the shallower troughs with height (L) 962 and width(W_(m)) 964 in conjunction with the top epitaxial silicon layer 804 ofthickness (W_(f)) 966. FIG. 119 shows a view 970 of the template in FIG.118 after highly selective etching of the sacrificial layer 802, forminggaps 972 thus allowing for release and removal of the 3-D TFSC substrate804 from the template. FIGS. 120 and 121 illustrate Y-Y cross-sectionalviews 980 and 990 of the released substrate 804 from FIG. 119. Thereleased substrate 804 has a base side 982, an emitter side 984. Thesubstrate 804 has dimensions of T_(st) 426 (silicon sidewall thicknessnear the base side of the hexagonal-prism vertical sidewalls), T_(sb)430 (silicon sidewall thickness near the emitter side of thehexagonal-prism vertical sidewalls), hexagonal-prism height 986, andtapered hexagonal-prism thin-film TFSC substrate sidewalls 988.Referring to the view 980 in FIG. 120, the base side 982 is shown on thetop and the emitter side 984 is shown on the bottom (TFSC substrate asreleased from the template). In the view 990 in FIG. 121, the base side982 is shown on the bottom and the emitter side 984 is shown on the top.FIG. 122 shows a Y-Y cross-sectional view 1000 of the template shown inFIG. 119 after releasing and separating/removing the embeddedhexagonal-prism single-aperture 3-D thin-film TFSC substrate with a rearbase layer (see template 510 in FIG. 47). The template 510 is ready formultiple reuse cycles.

FIGS. 123 through 125 illustrate Y-Y cross-sectional views of thetemplate 620 from FIG. 59 with square-based pyramid (shown as triangularcavity cross section) backside release channels 626, allowing forrelease etchant access from template backside and subsequent release ofthe 3-D thin-film TFSC substrate. FIG. 123 shows a view 1010 afterformation of a suitable sacrificial layer 1012 (made of porous silicon,silicon germanium, or another suitable material). Note that thethrough-wafer small-diameter (e.g., 500 to 5000 nanometers) holes 634are pinched off by porous (microporous or mesoporous) silicon. Poroussilicon sacrificial layer may be formed by epitaxial deposition of athin conformal layer of p-type silicon and its subsequent conversion toporous silicon (or alternatively by direct conversion of a thin surfacelayer of the template to sacrificial porous silicon). FIG. 124 shows aview 1020 after epitaxial deposition of in-situ-doped (e.g., boron-dopedfor p-type base) silicon 804. This step may fill the deep trenches andform the shallow troughs/trenches stacked over the epitaxy-filled deeptrenches. FIG. 125 shows a view 1030 after selective wet chemicaletching of sacrificial porous silicon layer 1012 (e.g., in HF/H₂O₂ orTMAH or another suitable selective etchant for porous silicon) torelease and remove the 3-D TFSC substrate 804 from the template 620.

FIGS. 126 through 128 illustrate Y-Y cross-sectional views of thetemplate 640 from FIG. 68 with square-based (rectangular-shaped)backside release channels 642, allowing for etchant access from thetemplate backside and subsequent release of the 3-D thin-film TFSCsubstrate. FIG. 126 shows a view 1040 after formation of a thinsacrificial layer 1012 of porous silicon. Note that the through-wafersmall-diameter holes 634 may be pinched off by porous (microporous ormesoporous) silicon. FIG. 127 shows a view 1050 after epitaxialdeposition of in-situ-doped silicon 804. This step may fill the deeptrenches and form the shallow troughs/trenches stacked over theepitaxy-filled deep trenches. FIG. 128 shows a view 1060 after selectivewet chemical etching of sacrificial porous silicon layer 1012 (e.g., inHF/H₂O₂ or TMAH or another suitable selective etchant) to release the3-D TFSC substrate 804 from the template 640.

FIGS. 129 through 131 illustrate Y-Y cross-sectional views of thetemplate 620 from FIG. 79 with backside release channels 644 alignedwith the bottoms of trenches, allowing for backside etchant access andsubsequent release of the 3-D thin-film TFSC substrate. FIG. 129 shows aview 1070 after formation of a thin (and relatively conformal)sacrificial layer 1012 of porous silicon. FIG. 130 shows a view 1080after epitaxial deposition of in-situ-doped silicon 804. This step mayfill the deep trenches and form the shallow troughs/trenches stackedover the epitaxy-filled deep trenches. FIG. 131 shows a view 1090 afterselective wet chemical etching of sacrificial porous silicon layer 1012(e.g., in HF/H₂O₂ or TMAH or another suitable selective wet etchant) torelease and remove the 3-D TFSC substrate 804 from the template 620.

The following section describes the use of 3-D thin-film TFSC substratesin fabrication of 3-D TFSCs.

FIG. 132 shows a schematic view 1100 of a hexagonal-prismsingle-aperture 1102 (on top 1104) 3-D TFSC substrate with thehexagonal-prism 3-D TFSC substrate rear side 1106 monolithicallyconnected to a relatively flat planar thin film 1108 (rear base layer).

FIG. 133 shows a schematic magnified top view 1110 of a regular(equilateral) hexagonal-prism 3-D thin-film TFSC substrate showing aplurality of prism unit cells 116. Each hexagonal unit cell 116 containshexagonal unit cell boundary points (H₁, H₂, H₃, H₄, H₅, and H₆) 1112,1114, 1116, 1118, 1120, 1122. FIG. 133 shows the hexagonal-prism 3-Dthin-film TFSC substrate sidewalls 808; the long diagonal dimension ofthe unit cell hexagon (d) 1124; and the short diagonal dimension of thehexagonal unit cell (h) 1126. In one embodiment, the hexagonal-prism 3-Dthin-film TFSC substrate sidewalls 808 are between 2 and 30 micronsthick.

FIG. 134A shows a Y-Y cross-sectional view 1130 of the hexagonal-prismsingle-aperture 3-D thin-film TFSC substrate with a rear base layershown in FIG. 133. FIG. 134B shows a Z-Z cross-sectional view 1140 ofthe hexagonal-prism 3-D thin-film TFSC substrate shown in FIG. 133.These FIGURES also show the hexagonal thin silicon walls 808monolithically attached to the rear base layer 1108.

FIGS. 135 through 140 show six different process flow embodiments ofthis disclosure for fabricating single-aperture hexagonal-prism 3-DTFSCs with rear base layers. While these process flow embodiments areoutlined for fabricating silicon-based TFSCs, the overall concepts andmethodologies may be extended and applied to other homojunction andheterojunction semiconductor materials (such as multicrystallinesilicon, polycrystalline silicon, CIGS, etc.). While the process flowsshown are for fabrication of 3-D c-Si TFSCs, the embodiments may beeasily adjusted and modified to fabricate silicon-based TFSCs usingpolysilicon, amorphous silicon, and/or multicrystalline silicon films.

FIGS. 135 through 140 show six different process flow embodiments 1150,1180, 1210, 1240, 1270, and 1300 of this disclosure for fabrication ofsingle-aperture hexagonal-prism 3-D TFSCs with rear base layers (i.e.,the honeycomb-prism structures have a monolithically attached thin baselayer). These embodiments may use one of the templates described andshown previously. These TFSC substrates may be fabricated usingtemplates with trenches with shallow wider trenches (shoulders) on topof deeper narrower trenches (or using deep trenches with flared outregions on top of the trenches, that is, trench widths larger on the topof the deep trenches compared to the trench widths in the lower sectionsof the deep trenches). These six embodiments all result insingle-aperture hexagonal-prism 3-D TFSCs with self-aligned selectiveemitter and base diffusion regions in silicon as well as self-alignedemitter and base contact metallization regions. These embodimentsinclude either detached rear mirrors (for instance, mirrors provided bysilver-coated copper or silver-coated aluminum pads on solar moduleprinted-circuit boards) or preferably integrated/attached rear mirrorsdeposited directly on the rear surface of the cell passivationdielectric (e.g., on thermal oxide) layer on the rear base layer. Boththe detached and integrated/attached mirrors may also serve as the baseinterconnect planes (electrically connected to the hexagonal basecontact metallization). In one embodiment, the material for ahigh-reflectivity mirror is silver (alternatively, aluminum may beused).

FIG. 135 shows a process flow 1150 for fabrication of single-aperturehexagonal-prism 3-D TFSCs with rear base layers using self-alignedselective plating metallization with boron-doped p⁺⁺ rear base contactsby selective base doping (besides selective emitter doping). Thishexagonal-prism 3-D TFSC with rear base layer uses a detached rearmirror (i.e., rear mirror is not an integrated layer directly depositedon the rear base layer). In step 1152, cell processing starts with asingle-crystal (or multicrystalline or polycrystalline) p-type (forn-type selective emitter), 3-D silicon TFSC substrate (e.g., a 3-D arrayof honeycomb hexagonal prisms). As with any other cells in thisdisclosure, the substrate doping polarity may be changed to n-type (forp-type selective emitter). The 3-D honeycomb prism TFSC substrate hasopen top apertures and no rear apertures (due to the rear base layer).There is a rear relatively flat base silicon layer monolithically andintegrally attached to the honeycomb-prism cell. Step 1154 involvesselectively coating the top ridges of the 3-D honeycomb prisms (in oneembodiment, the top 2 to 10 microns) with an n-type dopant source. Inone embodiment, this n-type dopant source is phosphorus. Selectivecoating may be done by self-aligned roller coating using paste/liquidsource, liquid-dip coating by dipping in a known liquid source depth,ink-jet coating, or spray coating. Next, the n-type dopant source layeris dried and cured (e.g., by thermal curing at 250° C. to 400° C. or UVirradiation). Step 1156 involves selectively filling the rear basetroughs on the substrate backside with a p-type liquid/paste dopantsource layer. In one embodiment, the p-type dopant source is boron.Selective filling may be done by boron source layer coating (e.g.,roller, spin-on, ink-jet, or spray coating) followed by selectiveetch-back (e.g., by solvent spin-on) to form filled troughs. Next, thep-type dopant source layer is dried and cured (e.g., by thermal curingat 250° C. to 400° C. or UV exposure). Step 1158 involves formingself-aligned selective emitter and base regions. The top n⁺⁺p emitterdiffusion contact, top aperture n⁺p selective emitter junctions, rearp⁺⁺ base contacts and selectively doped p⁺ base regions (the latter areoptional) are concurrently formed. This may be done using thermal annealin a diffusion furnace at 800° C. to 950° C. In one embodiment, the 3-DTFSC substrate is annealed while placed in an in-line diffusion furnace,or with stacks of 3-D TFSC substrates in face-to-face contact tofacilitate vapor-phase doping formation of n⁺ emitter and p⁺ base. Step1160 involves surface passivation (oxidation), where a thermal oxidelayer is grown, in one embodiment by steam oxidation (e.g., 10 to 200nanometers at 800° C. to 950° C.). Step 1160 may be merged into theprior diffusion step in multi-zone furnace, to be performed sequentiallyafter the selective emitter and base diffusion step. In one embodiment,the diffusion/oxidation steps result in selective emitter and emittercontact sheet resistance values of 80-150 Ω/square and 10-70 Ω/square,respectively. Step 1162 involves selective etching of the cured layersin preparation for self-aligned metallization. The dopant source layersare selectively etched (i.e., the n-type coating on top honeycomb prismridges and the p-type coating in rear filled troughs) with a suitabledielectric etchant (e.g., an HF-based etchant) with high selectivitywith respect to thermal oxide. This selectively strips the cured dopedand undoped dielectrics on the top and rear portions of the substrateand exposes silicon in those regions, while removing only a smallfraction of thermal oxide from other 3-D TFSC substrate regions. Step1164 involves self-aligned metallization (in one embodiment, byplating). The front and rear emitter and base metallized regions areconcurrently formed using selective electroplating and/or electrolessplating and/or galvanic plating to form single or multilayerhigh-conductivity metallized regions (silver, aluminum, nickel,titanium, cobalt, tantalum). For instance, the plated metal stack mayinclude a thin (50 to 500 nanometers) barrier and adhesion layer such asnickel (nickel) followed by a relatively thick (2 to 15 microns) layerof high-conductivity metal (silver or copper or aluminum). If ahigh-conductivity metal other than silver is used for the thickmetallization layer, a final flash coat of silver may be used to createa high-reflectivity surface coating in order to improve light reflectionand trapping into the 3-D cells (by the emitter metallization contact).Step 1166 involves an optional forming-gas anneal. A forming gas annealmay be performed (e.g., 350° C. to 450° C.) to reduce front and rearinterconnect resistance values and help with surface/bulk passivation.Step 1168 involves mounting honeycomb prism TFSC rear side (base side)onto a highly reflective (diffuse with a rough surface or specular witha smooth surface) rear mirror. This rear mirror may be made of ansilver-coated aluminum or copper foil and may also serve as the TFSCbase interconnect plane on a printed-circuit board (PCB) in a solarmodule. Step 1170 involves an optional step of depositing a passivationand ARC layer on mounted cells. In one embodiment, this passivation andARC layer is PVD or PECVD SiN_(x) with thickness between 50 and 200nanometers. Step 1172 involves proceeding with packaging the honeycombprism TFSCs in solar module assembly.

FIG. 136 shows an alternative process flow 1180 for fabrication ofsingle-aperture hexagonal-prism 3-D TFSCs with rear base layers usingself-aligned selective plating metallization without boron-doped p⁺⁺rear base contacts by selective base doping. The p⁺⁺ base contact dopingis performed by aluminum doping using aluminum from base contactmetallization and an anneal (contact firing process). As in FIG. 135,this process flow uses cured phosphorus source layer and a thermalanneal to form the n⁺ phosphorus-doped selective emitter regions and then⁺⁺ phosphorus-doped emitter contact diffusion regions. Thissingle-aperture hexagonal-prism 3-D TFSC with rear base layer also usesa detached rear mirror (i.e., rear mirror is not an integrated layerdirectly deposited on the rear base layer). Step 1182 (providing asubstrate) corresponds to step 1152 of FIG. 135; and step 1184(selective coating) corresponds to step 1154. Step 1186 (selectivefilling) involves selectively filling the rear base troughs on the TFSCsubstrate backside with an undoped sealant dielectric (e.g., oxideand/or nitride) using a dielectric liquid/paste source layer. Selectivefilling may be done by undoped dielectric source layer coating (e.g.,roller, spin-on, ink-jet, or spray coating) followed by selectiveetch-back (e.g., by solvent spin-on) to form filled troughs. Next, theundoped dielectric source layer is dried and cured (e.g., by thermalcuring at 250° C. to 400° C. or UV exposure). Step 1188 involvesself-aligned selective emitter and base. The top n⁺⁺p emitter contactand honeycomb top apertures n⁺p selective emitter junctions areconcurrently formed using thermal anneal at 800° C. to 950° C. In oneembodiment, the 3-D TFSC substrate may be annealed while placed in anin-line diffusion furnace, or with stacks of 3-D TFSC substrates inface-to-face contact to facilitate vapor-phase doping formation of n+pselective emitter and n⁺⁺p emitter contact regions. Step 1190 (surfacepassivation oxidation) corresponds to step 1160 of FIG. 135. Step 1192involves selective etching of the cured layers in preparation forself-aligned metallization. The dopant source layer (i.e., the n-typecoating on top honeycomb prism ridges) and the undoped dielectricfilling in rear filled troughs) are selectively etched with a suitableetchant (e.g., an HF etchant) with high selectivity with respect tothermal oxide. This selectively strips the cured dopant sourcedielectrics on the top and rear portions of the 3-D TFSC substrate andexposes silicon in those regions, while removing only a small fractionof thermal oxide from other 3-D TFSC substrate regions. Step 1194involves self-aligned metallization (embodiments include electrolessplating, galvanic plating, and/or electroplating). The rear basealuminum metallized regions are selectively formed by selectiveelectroplating or electroless plating. In one embodiment, this aluminumplating process limited to the rear base regions. Step 1196 involves ananneal or firing process to form rear aluminum-doped p⁺ contacts. Anoptional forming gas anneal (e.g., 350° C. to 450° C.) is performed toreduce interconnect resistance and help with surface/bulk passivation.Step 1198 involves self-aligned metallization (plating). A suitablehigh-conductivity metal such as silver or copper (e.g., 2 to 12 microns)is selectively/concurrently deposited on the top honeycomb ridges(emitter) and rear aluminum-filled troughs (aluminum metallized basecontacts) by plating. If necessary, the plating process may firstinvolve selective deposition of a suitable refractory metal barrier andadhesion layer (e.g., 50 to 200 nanometers nickel) followed by thedeposition of the thicker higher conductivity metal (silver and/orcopper). If necessary, the metallized regions are then flash coated witha thin layer of silver in order to establish a high optical/IRreflectivity (this step may be merged into the prior plating step). Ifsilver is used as the main metallization layer, then the top solverflash coating may not be needed. Step 1200 (mounting) corresponds tostep 1168 of FIG. 135; step 1202 (ARC layer) corresponds to step 1170;and step 1204 (proceeding with packaging) corresponds to step 1172.

FIG. 137 shows another alternative process flow 1210 for fabrication ofhexagonal prism 3-D TFSCs with rear base layers using self-alignedselective plating metallization with boron-doped p⁺⁺ rear base contactsby selective base doping (besides selective emitter doping). Thissingle-aperture hexagonal-prism 3-D TFSC with rear base layer uses anintegrated rear mirror which is directly deposited (e.g., silver oraluminum formed by sputtering, evaporation, or another method such asnon-selective plating) on the rear base passivation dielectric layer(the rear base passivation layer may be a thermal oxide layer).Optionally, the rear base passivation dielectric (e.g., oxide) layersurface may be treated (e.g., with plasma) to produce a rough dielectricsurface (e.g., with an RMS surface roughness in the range of tens tohundreds of nanometers) prior to mirror layer deposition. This mayproduce a diffuse integrated mirror directly on the rear surface of therear base layer passivation layer. Alternatively, the TFSC substrate mayalready be textured as a result of using a textured template, resultingin a diffuse rear mirror. Step 1212 (providing a substrate) correspondsto step 1182 of FIG. 136; and step 1214 (selective coating) correspondsto step 1184. Step 1216 involves selectively filling the rear basetroughs on the 3-D TFSC substrate backside with p-type (e.g., boron)liquid/paste dopant source. This may be done by boron source layercoating (e.g., roller, spin-on, ink-jet, or spray coating) followed byetch-back (e.g., by solvent spin-on) to form filled troughs. The layeris then dried and cured (using thermal curing at 250° C. to 400° C. orUV exposure). Step 1218 involves self-aligned selective emitter andbase. The top n⁺⁺p emitter contact, honeycomb top apertures n+pselective emitter junctions, the rear p⁺⁺ base contact and selectivelydoped p⁺ base regions are concurrently formed using thermal anneal at800° C. to 950° C. In one embodiment, the 3-D TFSC substrate is annealedwhile placed in an in-line diffusion furnace, or with stacks of 3-D TFSCsubstrates in face-to-face contact to facilitate vapor-phase dopingformation of n⁺ emitter and p⁺ base. Step 1220 (surface passivationoxide) corresponds to step 1190 of FIG. 136. Step 1222 involvesselective etching of the cured layers in preparation for self-alignedmetallization. The dopant source layers (i.e., the n-type coating on tophoneycomb prism ridges and the p-type coating in rear filled troughs)are selectively etched with a suitable dielectric etchant (e.g., anHF-based etchant) with a relatively high selectivity with respect tothermal oxide. This selectively strips the cured dopant layers on thetop and rear portions of the 3-D thin-film TFSC substrate and exposessilicon in those regions, while removing only a small fraction ofthermal oxide from other 3-D thin-film TFSC substrate regions (e.g.,thermal oxide coating remains on all selective emitter regions and rearbase backside regions outside the base contact area). Step 1224 involvesan optional plasma treatment step. A plasma treatment process may beperformed to roughen the thermal oxide layer on the substrate backside(for integrated diffuse rear mirror). Step 1226 involves self-alignedmetallization (plating). The front and rear emitter and base metallizedregions are concurrently formed using selective electroplating and/orelectroless plating and/or galvanic plating to form single or multilayerhigh-conductivity metallized regions (silver, aluminum, nickel,titanium, cobalt, tantalum). For instance, the plated metal stack mayinclude a thin (50 to 500 nanometers) barrier and adhesion layer such asnickel (nickel) followed by a relatively thick (2 to 15 microns) layerof high-conductivity metal (silver or copper or aluminum). If ahigh-conductivity metal other than silver is used for the thickmetallization layer, a final flash coat of silver may be used to createa high-reflectivity surface coating in order to improve light reflectionand trapping into the 3-D cells (by the emitter metallization contact).Step 1228 involves an optional forming gas anneal step is (e.g.,performed at 350° C. to 450° C.) to reduce front and rear interconnectresistance values and help with surface/bulk passivation. Step 1230involves addition of an integrated rear cell mirror. A thin (e.g., 50 to1000 nanometers) layer of high-reflectance metal (silver and/oraluminum, among others) is deposited on TFSC substrate backside (e.g.,by PVD, non-selective plating, or evaporation). This thin layer alsoserves as base interconnect plane. Step 1232 involves an optional stepof depositing an ARC (e.g., 50 to 200 nanometers PVD or PECVDhydrogenated SiN_(x) or AlO_(x)) layer on substrate frontside. This stepis may be performed either before or after mounting the cells in themodule assembly. Step 1234 (proceeding with packaging) corresponds tostep 1204 of FIG. 136.

FIG. 138 shows another alternative process flow 1240 for fabrication ofsingle-aperture hexagonal-prism 3-D TFSCs with rear base layers usingself-aligned selective plating metallization without boron-doped p⁺⁺rear base contacts by selective base doping. The p⁺⁺ base contact dopingis performed by aluminum doping (aluminum contact firing) using aluminumfrom base contact metallization and an anneal (firing process). Thisprocess flow uses cured phosphorus source layer and a thermal anneal toform the n⁺ phosphorus-doped selective emitter regions and the n⁺⁺phosphorus-doped emitter contact regions (the latter underneath thecured phosphorus doping layer). This single-aperture hexagonal-prism 3-DTFSC with rear base layer uses an integrated rear mirror which isdirectly deposited (embodiments include silver and/or aluminum depositedby PVD, evaporation, or non-selective plating) on the rear basepassivation dielectric layer. As an option, the rear base passivationdielectric (e.g., oxide) layer surface may be treated (e.g., withplasma) to produce a roughened dielectric surface (e.g., with an RMSsurface roughness of roughly tens to hundreds of nanometers) prior tomirror layer deposition. This produces a diffuse integrated mirrordirectly on the rear surface of the rear base layer passivation layer.Alternatively, the TFSC substrate rear base layer may be pre-textured bya textured template surface (thus, eliminating the need for such plasmatreatment). Step 1242 (providing a substrate) corresponds to step 1212of FIG. 137; and step 1244 (selective coating) corresponds to step 1214.Step 1246 (selective filling) corresponds to step 1186 of FIG. 136; step1248 (self-aligned selective emitter and base) corresponds to step 1188of FIG. 136; step 1250 (surface passivation) corresponds to step 1190 ofFIG. 136; and step 1252 (etch) corresponds to step 1192 of FIG. 136.Step 1254 (optional plasma treatment step as part of self-alignedmetallization) corresponds to step 1224 of FIG. 137. Step 1256 (plating)corresponds to step 1194 of FIG. 136; step 1258 (anneal) corresponds tostep 1196 of FIG. 136; and step 1260 (plating) corresponds to step 1198of FIG. 136. Step 1262 (integrated mirror) corresponds to step 1220 ofFIG. 137; step 1264 (ARC layer) corresponds to step 1222 of FIG. 137;and step 1266 (proceeding with packaging) corresponds to step 1224 ofFIG. 137.

FIG. 139 shows another alternative process flow 1270 for fabrication ofsingle-aperture hexagonal-prism 3-D TFSCs with rear base layers usingself-aligned fire-through metallization with boron-doped p⁺⁺ rear basecontacts by selective base doping (besides selective emitter doping).This hexagonal-prism 3-D TFSC with rear base layer uses a detached rearmirror in module assembly (i.e., rear mirror is not an integrated layerdirectly deposited on the rear base layer). Step 1272 (providing asubstrate) corresponds to step 1212 of FIG. 137; step 1274 (selectivecoating) corresponds to step 1214; step 1276 (selective filling)corresponds to step 1216; and step 1278 (self-aligned selective emitterand base) corresponds to step 1218. Step 1280 involves formation ofsurface passivation and ARC. The top (emitter phosphorus) dopant sourcelayer, rear (base boron) dopant source layer, and any native oxide arestripped using a suitable etchant (e.g., using HF etchant). A thin oxidelayer (e.g., 5 to 100 nanometers) is grown by steam oxidation (e.g., 3to 300 nanometers at 800° C. to 950° C.). This thermal oxidation step isoptional. Next, an ARC layer (e.g., 3 to 100 nanometers hydrogenatedSiN_(x)) is deposited by PECVD or PVD, with passivation layer formed onprism top and sidewalls (coverage on the cell rear is optional). ThePECVD or PVD SiN_(x) (or AlO_(x)) also provides H passivation of the 3-DTFSC substrate. In one embodiment, the diffusion/oxidation steps resultin selective emitter and emitter contact diffusion sheet resistancevalues of 80-150 Ω/square and 10-70 Ω/square, respectively. Step 1282involves self-aligned metallization (metal coat). The top portions ofthe honeycomb prisms are selectively coated (to a height equal to orless than the dopant source layer) with metal (in one embodiment,silver) liquid or paste using self-aligned roller, inkjet, liquid dip,or spray coating. Next, this layer is dried and cured (250° C. to 400°C. or UV). The rear base troughs on the substrate backside are thenselectively filled with metal liquid or paste (silver and/or aluminum).This may be done by roller, spin-on, ink-jet, or spray coating followedby etch-back (e.g., by solvent spin-on or selective cell backsideetchback) to form filled troughs. This layer is then dried and cured(250° C. to 400° C. or UV). Step 1284 involves self-alignedmetallization (fire-through). The cell front (silver) and rear (aluminumand/or silver) metallized regions are formed by firing through theoxide/PECVD (or PVD) SiN_(x) layers. Step 1286 involves an optionalself-aligned plating metallization step. A layer of silver or copper(e.g., roughly 1 to 5 microns) is selectively/concurrently deposited onthe metallized top honeycomb ridges (emitter) and rear honeycomb ridges(base) by plating. If necessary, a refractory metal barrier layer suchas nickel may be deposited by plating before copper or silver plating.Next, the metallized regions are flash coated with silver. Step 1288(optional FGA) corresponds to step 1228 of FIG. 137. Step 1290(mounting) corresponds to step 1200 of FIG. 136. Step 1292 (proceedingwith packaging) corresponds to step 1266 of FIG. 138.

FIG. 140 shows another alternative process flow 1300 for fabrication ofsingle-aperture hexagonal-prism 3-D TFSCs with rear base layers usingself-aligned fire-through metallization and with boron-doped p⁺⁺ rearbase contacts formed by selective base doping (besides selective emitterdoping). This hexagonal-prism 3-D TFSC with rear base layer uses anintegrated (attached) rear mirror which is directly deposited (e.g.,silver or aluminum by PVD or evaporation or non-selective plating) onthe rear base passivation dielectric layer. As an option, the rear basepassivation dielectric (e.g., oxide) layer surface may be treated (e.g.,with plasma) to produce a rough dielectric surface (e.g., with an RMSsurface roughness of roughly tens to hundreds of nanometers) prior tomirror layer deposition. This may produce a diffuse integrated mirrordirectly on the rear surface of the rear base layer passivation layer.Alternatively, a textured TFSC substrate base layer may be formed byusing a textured template (thus, eliminating the need for an optionalplasma treatment step). Step 1302 (providing a substrate) corresponds tostep 1272 of FIG. 139; step 1304 (selective coating) corresponds to step1274; step 1306 (selective filling) corresponds to step 1276; step 1308(self-aligned emitter and base) corresponds to step 1278; and step 1310(surface passivation and ARC) corresponds to step 1280. Step 1312involves an optional plasma treatment step to roughen the thermal oxidelayer on the substrate backside (for integrated diffuse rear mirror).Step 1314 (metal coat) corresponds to step 1282 of FIG. 139; step 1316(fire-through) corresponds to step 1284; step 1318 (optional FGA)corresponds to step 1288; and step 1320 (plating) corresponds to step1286. The plating and FGA process steps are reversed. Step 1322(integrated mirror) corresponds to step 1262 of FIG. 138; and step 1324(proceeding with packaging) corresponds to step 1266.

In regard to the n-type (e.g., phosphorus) dopant liquid/paste coveringthe top portion of the hexagonal ridges, a single furnace anneal processin a diffusion furnace (e.g., at roughly 800° C. to 950° C.) producesmore heavily-doped regions with higher surface phosphorus concentrationson the top silicon hexagonal ridges directly in contact with andunderneath the cured n-type dopant solid source layer compared to otherregions not covered with the cured dopant source layer. The TFSCsubstrates may be processed with the emitter side facing down through anin-line diffusion furnace. Through vapor-phase transport of thevaporized dopant source to the adjacent frontside regions within thehexagonal prism unit cell cavities, the furnace anneal concurrentlydopes the remaining frontside surface regions not covered with the soliddopant source layer with phosphorus with smaller surface concentration(e.g., 1×10¹⁹ to 5×10¹⁹ cm⁻³), thus, creating self-aligned selectiveemitter regions. These less heavily doped regions with higher sheetresistance values (in one embodiment, in the range of 100 Ω/square to150 Ω/square) improve the blue response of the 3-D TFSC, while the moreheavily doped honeycomb ridges may minimize the frontside emittercontact resistance of the 3-D TFSC. Similarly, the same furnace annealprocess produces more heavily doped p⁺-doped hexagonal prism diffusedbased contacts for low base contact resistance. For hexagonal-prism 3-DTFSCs with rear base layers, the remaining rear base layer rear surfacebase regions are less heavily doped on the surface, resulting inselective base doping (and a back-surface field or BSF region).

FIG. 149 shows a 3-D view 1330 of multiple adjacent prism unit cellsfrom regular hexagonal prism TFSC of this disclosure, after cellfabrication, including self-aligned base and emitter contactmetallization. The dark region on the top 1332 of the unit cell is theself-aligned emitter contact metal; the rear 1334 of the unit cell isthe self-aligned base contact metal. The prism sidewall surfaces aredoped to form the selective emitter junctions (e.g., shallow n⁺pjunctions with a junction depth of 0.2 to 0.5 micron in boron-dopedsilicon base). The view 1330 is after mounting the cell rear base side(base contact metal regions 1334) onto a rear mirror 1336. The rearmirror 1336 may be made of a suitable planar material (e.g., copper oraluminum) flash-coated with silver or aluminum (e.g., using plating).The rear mirror may be copper interconnect pads or islands (plated withsilver and/or aluminum) on a printed-circuit board (PCB) within a solarmodule assembly. Thus, the rear mirrors may also serve as baseelectrical interconnect planes. The rear base contact metal regions 1334may be soldered or glued (or simply placed) onto the plated copper padson the solar module PCB assembly. There is an optional step of silverflash coating of the assembled module structure to coat the solder jointsurfaces with optically reflective silver (silver).

FIGS. 142A through 148 show various cross-sectional views ofhexagonal-prism unit cells with rear base layers, with detached orintegrated/attached rear mirrors. These FIGURES correspond to the cellfabrication process flow embodiments outlined in FIGS. 135-140. The celldoping polarities may be inverted (e.g., phosphorus-doped base and p⁺nselective emitter). While depicted for c-Si cells, this cell structuremay also be applied to polysilicon, amorphous silicon, and non-Siabsorber TFSCs. The substrates shown have tapered prism sidewalls(narrower emitter and wider base). Alternatively, the substrate may havevertical prism sidewalls.

FIG. 142A shows a Y-Y cross-sectional view 1340 of a unit cell within asingle-aperture hexagonal-prism 3-D thin-film TFSC substrate with a rearbase layer (released and removed from its template) before cellfabrication. For subsequent n⁺p selective emitter formation, thehexagonal-prism sidewalls are in-situ-doped with boron to form the baseregion at the time of 3-D thin-film TFSC substrate fabrication. Thesidewalls are doped with boron (in one embodiment, at the time ofsilicon deposition into the template), either uniformly or in a gradedprofile, more lightly doped at the prism sidewall surface and moreheavily doped towards the sidewall vertical center axis. Similarly, thehexagonal-prism rear base layer is in-situ-doped at the time of 3-D TFSCsubstrate fabrication. The base layer is doped with boron, eitheruniformly or in a graded profile, more lightly doped at the rear baselayer top surface and more heavily doped towards the rear base layerrear surface, creating a built-in back-surface-field effect in the rearbase layer, improving the cell performance. The prism top (emitter side)ridges 1342 are used for emitter contact diffusion and metal contactformation and the hexagonal troughs 1344 for base contact diffusion andburied metal contact formation.

FIG. 142B shows a Y-Y cross-sectional view 1350 of a unit cell withinthe hexagonal prism 3-D TFSC of this disclosure (using the hexagonalprism 3-D TFSC substrate with a rear base layer as shown in FIG. 142A)after self-aligned formation of: selective emitter regions 1352 (e.g.,less heavily-doped with phosphorus, n⁺ selective emitter on thehexagonal prism sidewall surfaces as shown); heavily-doped emittercontact regions 1354 with coverage height L_(e) 1356 (e.g., moreheavily-doped with phosphorus, n⁺⁺ doped emitter contact regions on thehexagonal prism top hexagonal ridges as shown); selective base regions1358 on the rear surface of the rear base layer (e.g., lessheavily-doped with boron, p⁺ selective base on the rear base layer rearsurface as shown); and heavily-doped (boron-doped p⁺⁺) base contactdiffusion regions 1360 in the rear base layer trenches/troughs (e.g.,more heavily-doped with boron, p⁺⁺ doped base contact regions). Thecured solid dopant source layers for emitter 1355 and base regions 1362are shown as dark segments on the top hexagonal-prism ridges and withinthe rear base rear filled trenches (troughs), respectively.

FIG. 143A shows a Y-Y cross-sectional view 1370 after the cured n-typeand p-type dopant layers have been removed and before the thermaldiffusion process. FIG. 143B shows a Y-Y cross-sectional view 1380 afterformation of surface passivation and anti-reflection coating (thermalSiO₂ and/or PVD or PECVD SiN_(x) or AlN_(x) ARC) dielectric layers 1382.Note L_(e) 1384 and cured boron doped glass 1386. FIG. 144A shows a Y-Ycross-sectional view 1390 after formation of emitter 1392 and base 1394contact metals (silver, aluminum, copper, etc.) by fire-through and/orselective plating. FIG. 144B shows a Y-Y cross-sectional view 1400 afterthe addition of a detached highly reflective rear specular or diffusemirror 1336 (e.g., silver or aluminum coating on a base interconnectplane on a PCB in the solar module assembly; the mirror may contact therear base contacts as shown).

FIG. 145 shows a Y-Y cross-sectional view 1410 after the addition of anintegrated/attached highly reflective thin rear specular or diffusemirror (e.g., a thin layer 1412 of silver or aluminum coating depositedby PVD or plating on the rear base layer rear surface as shown; fordiffuse mirror, the dielectric layer on the rear base layer rear surfaceis roughened by a surface roughening process such as a plasma treatmentor ion bombardment before mirror metal deposition). Alternatively, thebase layer may already be textured by a pre-textured template from whichthe TFSC substrate is released.

FIG. 146 shows a Y-Y cross-sectional view 1420 of the TFSC in FIG. 145(showing multiple prism unit cells). The TFSC includes an integrated(attached) high-reflectivity rear mirror 1412, made of silver oraluminum, which may be deposited (e.g., by PVD or plating or evaporationor another coating technique such as roller or spray coating followed bycuring) on the rear passivation oxide (and ARC) layer(s) formed on therear surface of the rear base layer. Rear mirror 1412 is also the baseinterconnect plane, electrically connecting to the self-alignedhexagonal base contacts 1422 (e.g., silver and/or aluminum and/or copperor other metals) on the rear base layer. The rear mirror may bedeposited on a smooth or roughened rear base dielectric layer surface(for specular or diffuse mirror, respectively). Again, the base layermay already be textured by a pre-textured template from which the TFSCsubstrate is released.

FIG. 147 shows a Y-Y cross-sectional view 1430 of the TFSC in FIGS. 145and 146, with multiple prism unit cells shown. The TFSC includes adetached diffuse high-reflectivity rear mirror 1432, made of silver oraluminum (mirror coating), placed below the rear surface of the rearbase layer. This FIGURE shows the module assembly interconnect planeplaced at a spacing of S 1434 below the rear surface of the rear baselayer, where S may be in the range of 0 (i.e., interconnect plane incontact with the rear base layer rear surface) up to roughly H (where His the height of the hexagonal prism unit cell and may be in the rangeof 100 to 500 microns). In this latter structure, the rear mirror is notelectrically connected to the base contact metal. Without the integratedmirror, the interconnect plane with a suitable coating (in oneembodiment, silver) may serve as a detached rear mirror.

FIG. 148 shows a schematic Y-Y cross-sectional view 1440 of the TFSC inFIG. 144A, with multiple prism unit cells shown. The TFSC includes adetached diffuse high-reflectivity rear mirror 1442, made of silver oraluminum (mirror coating), placed below the rear surface of the rearbase layer. This FIGURE shows the mirror placed at a spacing of S belowthe rear surface of the rear base layer, where S may be in the range of0 (i.e., mirror in contact with the rear base layer rear surface) up toroughly H (where H is the height of the hexagonal prism unit cell andmay be in the range of 100 to 500 microns). In this structure, the rearmirror is electrically connected to the base contact metal. Thus, therear mirror shown here also serves as the base interconnect plane.

The above process steps may be performed on integrated in-line processequipment. For example, FIG. 149 shows a view 1450 of a setup forperforming the two process steps of liquid/paste coating and UV or IRcuring prior to furnace anneal, allowing for subsequent formation ofselective emitter and base regions after anneal in an in-line diffusionfurnace. This integrated in-line process equipment allows forself-aligned formation of dopant liquid or paste coating on the 3-D TFSCsubstrate hexagonal-prism top ridges and hexagonal-prism rear ridges byroller coating. Roller coating may be performed using anatmospheric-pressure, belt-driven coating and curing equipmentintegrated in line with a diffusion furnace. In one embodiment, the topridges are coated with n-type dopant liquid/paste; the rear ridges arecoated with p-type dopant liquid/paste.

The 3-D TFSC substrate 1452 is shown moving in 1454 on input conveyorbelt 1456. The rotating top rollers 1458, with top roller pads 1460,apply a controlled downward force to coat the top hexagonal prism ridgeswith n-type paste. The rotating rear rollers 1462, with rear roller pads1464, apply a controlled upward force to coat the rear hexagonal prismridges with p-type paste. Multilayer materials may be coated on eachside of the 3-D TFSC substrate by applying (or flowing) a differentliquid or paste material to each roller on the top 1458 and/or rear 1462set of rollers. The 3-D TFSC substrate 1452 next moves into the curingarea where the dopant liquid/paste layers are concurrently formed usinga curing lamp 1466 which uses IR or UV curing beams 1468. The 3-D TFSCsubstrate 1452 is next shown moving out 1470 to the output conveyor belt1472, which may move the substrate 1452 to an in-line diffusion furnace,where the n⁺ and p⁺ contacts and selective emitter regions areconcurrently formed.

A similar roller coater setup may be properly configured and used forapplying metal liquid/paste coatings (e.g., silver and/or aluminumliquid or paste sources), curing the metal liquid/paste source, andperforming subsequent thermal anneal in an in-line atmospheric furnace(resistively-heated or lam-heated furnace) for fire-throughmetallization in order to form the emitter and base contactmetallization (and whenever applicable, also to form the aluminum-dopedp⁺⁺ base contact regions).

FIG. 150 shows a view 1480 of an alternative setup design to perform thesame processes as the roller coater/curing/furnace setup of FIG. 149.The setup in FIG. 150 may be used for self-aligned formation of dopantsource liquid/paste coating on the 3-D TFSC substrate top ridges andhexagonal-prism rear ridges by angled spray coating. This setup also mayutilize an in-line atmospheric-pressure coating and curing and diffusionequipment configuration which can be easily integrated with an in-linediffusion furnace. As with the roller coater setup in FIG. 149,multilayer materials may be coated on each side of the substrate byusing multiple sets of spray nozzles connected to different liquidsources (not shown here) and applying (or flowing) a different liquidsource material to each nozzle on the top and/or rear set of spraynozzles. This is an alternative technique to the roller coating systemshown in FIG. 149. In one embodiment, the top ridges are coated withn-type dopant liquid/paste (such as phosphorus); the rear ridges arecoated with p-type dopant liquid/paste (such as boron). Referring toFIG. 150, the 3-D TFSC substrate 1452 is shown moving in 1454 on inputconveyor belt 1456. Angled nozzles 1422 spray n-type dopant liquid ontothe surface at a sharp angle with respect to the surface (nozzles coverwafer width). This n-type dopant liquid comes from an n-type liquiddopant source and nozzle reservoir/pump 1424. Angled nozzles 1486 sprayp-type dopant liquid onto the surface at a sharp angle with respect tothe surface (nozzles cover wafer width). This p-type dopant liquid comesfrom a p-type liquid dopant source and nozzle pump 1488. The 3-D TFSCsubstrate 1452 next moves into the curing area where the dopantliquid/paste layers are concurrently formed using a curing lamp 1466which uses IR or UV curing beams 1468. The 3-D TFSC substrate 1452 isnext shown moving out 1470 to the output conveyor belt 1472, which maymove the substrate 1452 to an in-line diffusion furnace, where the n⁺and p⁺ contacts and selective emitter regions are concurrently formed.

The angled spray technique limits the vertical height of theliquid/paste coating to a portion of the hexagonal ridges and preventsthe liquid source from coating the inner parts of the hexagonal prismcavity sidewalls and/or rears. This type of in-line (or another drivemethod) processing system may also be used for applying metal sourceliquid (e.g., silver and/or aluminum source liquid) for fire-throughmetallization applications as well as applying liquid etchant forselective etching of dielectrics (e.g., oxide and/or solid dopant sourcelayer) from the top and/or rear hexagonal prism ridges.

FIG. 151 shows a view 1490 of another alternative setup to perform thesame processes as the in-line roller coater/curing setup of FIG. 149 andthe in-line spray coater/curing setup of FIG. 150. The setup in FIG. 151may be used for self-aligned formation of dopant liquid/paste coating onthe 3-D TFSC substrate hexagonal top ridges and hexagonal prism rearridges by liquid-dip coating. This setup also may utilize an in-lineatmospheric-pressure coating and curing equipment configuration to beattached to the input stage of an in-line diffusion (or fire-through)furnace.

In one embodiment, the top ridges are coated with n-type dopantliquid/paste (such as phosphorus); the rear ridges are coated withp-type dopant liquid/paste (such as boron). The 3-D TFSC substrate 1452is shown moving in 1454 on input conveyor belt 1456. Liquid filmdispenser containing n-type liquid dopant source 1492 applies acontrolled thickness n-type liquid dopant film 1494. This n-type dopantliquid comes from n-type liquid dopant source and liquid level and depthcontroller 1496. Liquid film dispenser containing p-Type liquid dopantsource (with peripheral air levitation) 1498 applies a controlledthickness p-type liquid dopant film 1500. This p-type dopant liquidcomes from p-type liquid dopant source and liquid level and depthcontroller 1502. The 3-D TFSC substrate 1452 next moves into the curingarea where the dopant liquid/paste layers are concurrently formed usinga curing lamp 1466 which uses IR or UV curing beams 1468. The 3-D TFSCsubstrate 1452 is next shown moving out 1470 to the output conveyor belt1472, which may move the substrate 1452 to an in-line diffusion furnace,where the n⁺ and p⁺ contacts and selective emitter regions areconcurrently formed.

As in the setups in FIGS. 134 and 135, multilayer materials may becoated on each side of the 3-D TFSC substrate by using multiple sets ofliquid-dip applicators (not shown here) and applying (or flowing) adifferent liquid source material to each liquid-dip applicator on thetop and/or rear set of applicators. This type of processing system mayalso be used for applying metal liquid for fire-through metallization aswell as applying liquid etchant for selective etching of dielectrics(e.g., oxide and/or solid dopant source layer) from the top and/or rearhexagonal prism ridges.

FIG. 152 shows a view 1510 of a hexagonal-prism dual-aperture 3-Dthin-film TFSC substrate after release from a template where thehexagonal-prism 3-D TFSC substrate has both top (emitter) open apertures1102 and rear (base) apertures (not shown). The top ridges 1104 are usedfor n⁺⁺ emitter diffusion and contact metal, while the rear ridges 1106are used for p⁺⁺ base diffusion and contact metal. FIG. 153A shows a Y-Ycross-sectional view 1530 of a hexagonal prism 3-D thin-film TFSCsubstrate shown in FIG. 152. Note that the 3-D thin-film TFSC substratehas height 1132 (typically a value in the range of 100 to 350 microns).FIG. 153B shows a Z-Z cross-sectional view 1540 of hexagonal-prism 3-Dthin-film TFSC substrate shown in FIG. 152. Note that the Z-Z view showsthin-film walls with width d/2 182 (compared to the hexagonal aperturelong diagonal diameter d).

FIGS. 154 through 159 show six different process flow embodiments ofthis disclosure for fabricating dual-aperture hexagonal-prism 3-D TFSCswithout rear base layers. While these process flow embodiments areoutlined for fabricating silicon-based TFSCs, the overall concepts andmethodologies may be extended and applied to other homojunction andheterojunction semiconductor materials (such as multicrystallinesilicon, polycrystalline silicon, CIGS, etc.). While the process flowsshown are for fabrication of 3-D c-Si TFSCs, the embodiments may beeasily adjusted and modified to fabricate silicon-based TFSCs usingpolysilicon, amorphous silicon, and/or multicrystalline silicon films.

All of the fabrication process flow embodiments of FIGS. 154 to 159result in dual-aperture hexagonal-prism 3-D TFSCs without rear baselayers and with rear diffuse or specular reflective mirrors. The rearmirrors may be metal (e.g., silver-coated copper or silver-coatedaluminum) pads on thin printed-circuit boards used for assembling TFSCsinto packaged solar modules.

FIG. 154 describes a first process flow 1550 for fabricatingdual-aperture hexagonal-prism 3-D TFSCs without rear base layers. Thisflowchart describes the process flow for fabrication of hexagonal-prism3-D TFSCs using self-aligned fire-through metallization with an optionalforming-gas anneal (FGA) process being performed after an optionalselective silver (silver) or selective copper (copper) plating process.This process flow uses roller coating (or spray coating, inkjet spraycoating, or liquid-dip coating) and curing of liquids or pastes ofdopant and metal materials to form the selectively doped emitter regionsas well as emitter and base contact metallization regions. In thisembodiment, while the heavily n⁺⁺ (phosphorus) doped emitter contactregions (for subsequent emitter contact metallization) are formed byusing a cured solid phosphorus dopant source layer formed over thehexagonal-prism top ridges (after thermal anneal), the heavily p⁺⁺ dopedbase contact regions (for subsequent base contact metallization) areformed by aluminum doping of silicon during the fire-throughmetallization process (on the rear hexagonal ridges coated with curedaluminum liquid/paste layer).

Referring to FIG. 155, step 1552 starts with a dual-aperture 3-Dthin-film TFSC substrate (e.g., with in-situ p-type base doping formedduring TFSC substrate fabrication), with open top and rear apertures(i.e., no rear flat base silicon layer). In step 1554, the top and rearportions of the 3-D substrate are selectively coated with n-type dopant(e.g., phosphorus-containing liquid or paste) and an undoped cappingdielectric sealant (e.g., oxide), respectively, from liquid sources. Inone embodiment, these coatings are between 2 and 10 microns in height oneach side of the 3-D TFSC substrate and the n-type dopant is phosphorus.The rear (base side hexagonal ridges) portion of the substrate isselectively coated with undoped capping dielectric sealant such as anoxide. These coatings are applied using liquid or paste sources. Variouscoating embodiments include self-aligned 2-sided roller coating usingpaste/liquid sources, liquid-dip coating by controlled dipping in acontrolled liquid source depth, ink-jet coating, spray coating, oranother suitable coating method. The dopant source and oxide layers arethen dried and cured at 150° C. to 400° C. using flash IR lamp or UVradiation. Step 1556 involves formation of self-aligned selectiveemitter junctions and heavily doped emitter contacts. The top heavilydoped n⁺⁺p emitter contact and honeycomb prism sidewall n⁺p selectiveemitter junctions are concurrently formed by a thermal anneal process(in one embodiment, at 800° C. to 950° C.). Drying/curing and emitteranneal may both be performed sequentially in a belt furnace. The 3-Dthin-film TFSC substrates, in one embodiment arranged with pairs orstacks of 3-D substrates in face-to-face contact, may be annealed in aheated belt furnace in order to facilitate gas-phase doping formation ofn⁺ selective emitter junctions. The rear cap dielectric (e.g., oxide)coating prevents phosphorus doping of the base contact regions. Step1558 involves surface passivation and anti-reflection coating (ARC).First, the top dopant source layer, rear undoped oxide layer, and nativeoxide are stripped using hydrofluoric acid or another suitable etchant(e.g., dilute HF). Next, a thin oxide layer is grown, in one embodimentby steam oxidation. In one embodiment, this layer is between 3 and 300nanometers, performed at 800° C. to 950° C. Next, a passivation and ARClayer is deposited by PECVD or PVD, with coating formed on prism top(emitter side) and selective emitter sidewalls (coating on the base sideis optional). In one embodiment, this layer is between 3 and 150nanometers (preferably between 50 and 100 nanometers) of SiN_(x) orAlO_(x). PECVD SiN_(x) or AlO_(x) also provides H passivation of the 3-DTFSC substrate. In one embodiment, the diffusion/oxidation steps resultin selective emitter junction and emitter contact sheet resistancevalues of 80-150 Ω/square and 10-70 Ω/square, respectively. Step 1560involves self-aligned metallization (application of a metal coat). Thetop portions of the substrate is selectively coated (to a height equalto or less than the dopant source layer; in one embodiment, between 5and 20 microns) with metal (in one embodiment, silver) paste or liquidusing self-aligned liquid-dip coating, roller coating, inkjet coating,or spray coating. Next, this metal coat is dried and cured. Next, therear portions of the TFSC substrate are selectively coated (to a heightequal to or less than the rear cap dielectric layer) with metal (in oneembodiment, aluminum) to form p⁺⁺ base contact and metallization)paste/liquid by self-aligned liquid-dip coating, roller coating, inkjetcoating, or spray coating. Next, these metal coatings are dried andcured. Step 1562 involves self-aligned metallization (fire-through). Thecell front (e.g., silver) and rear (aluminum) metallized regions areformed by firing through the passivation layer or layer stack (oxideand/or PVD or PECVD SiN_(x) layers). Step 1564 involves an optionalself-aligned metallization step in which silver and/or copper isselectively/concurrently deposited (e.g., 1 to 5 microns) on themetallized top honeycomb ridges (emitter) and rear honeycomb ridges(base) by plating (such as electroless plating or galvanic plating),with further flash coating of the metallized regions with silver. Ifdesired, the plating process may also include an initial layer of arefractory metal barrier such as nickel (prior to silver and/or copperplating). Step 1566 involves another optional self-aligned metallizationstep. A forming gas anneal (FGA) is performed (e.g., at 300° C. to 450°C.) to reduce front and rear interconnect resistance values and helpwith surface/bulk passivation. This forming gas annealing step may notbe needed due to the prior metallization firing step. Step 1568 involvesmounting dual-aperture honeycomb-prism TFSC rear side (base side) onto ahighly reflective rear mirror. This rear mirror may be a diffuse mirrorwith a rough surface or specular with a smooth surface (a diffuse mirroris usually preferred). The rear mirror may be made of an silver-coatedaluminum or copper pad (foil) and may also serve as the TFSC baseinterconnect plane on a printed-circuit board placed in a solar moduleassembly. In step 1570, the honeycomb prism TFSCs are packaged in solarmodule assembly.

FIG. 155 shows an alternative process flow 1580 for fabricatingdual-aperture hexagonal-prism 3-D TFSCs without rear base layers. Exceptfor the order of the optional FGA and optional plating processes in theflow, the process flows of FIGS. 154 and 155 are otherwise similar andcomparable. Specifically, steps 1582 to 1592 in FIG. 155 are comparableto steps 1552 to 1562 in FIG. 154. Step 1594 in FIG. 155 involves theoptional FGA, before step 1596 which involves the optional platingprocess. Steps 1598 (mounting) and 1600 (proceeding with packaging) arethe same as steps 1568 and 1570 in FIG. 154.

FIG. 156 shows an alternative process flow 1610 for fabricatingdual-aperture hexagonal-prism 3-D TFSCs without rear base layers. Theprocess flow of this embodiment uses a fire-through metallizationprocess to form the self-aligned emitter and base contact metallization.This flowchart describes the process flow for fabrication ofhexagonal-prism 3-D TFSCs using self-aligned fire-through metallizationwith boron-doped p⁺⁺ rear base contacts as well as phosphorus-doped n⁺⁺emitter contacts (including selective emitter doping on the 3-D prismsidewalls). While the p⁺⁺ base contacts in the embodiments of FIGS. 154and 155 are formed by aluminum doping during fire-through metallization,as mentioned above, the p⁺⁺ base contacts in the embodiment of FIG. 156are formed by boron doping from a cured boron source layer. Step 1612(providing a substrate) of FIG. 156 corresponds to step 1552 of FIG.155. Step 1614 involves selectively coating the top and rear portions ofthe 3-D honeycomb-prisms (in one embodiment, the top and rear 2 to 10microns) with phosphorus (i.e., n-type dopant) liquid/paste source ontop as well as boron (i.e., p-type dopant) and undoped dielectric (e.g.,oxide) liquid/paste sources on rear (boron layer capped by dielectricsealant such as oxide). This selective coating may be done usingself-aligned 2-sided roller coating using paste/liquid sources,liquid-dip coating by dipping in a controlled liquid source depth,ink-jet coating, or spray coating. Next, the dopant source layers anddielectric (oxide) cap layer are dried and cured (250° C. to 400° C.thermal curing or UV). Step 1616 involves formation of self-alignedselective emitter junction as well as the heavily doped emitter and basecontact diffusion regions. The top n⁺⁺p emitter contact diffusion andhoneycomb-prism sidewall n+p selective emitter junctions as well as rearp⁺⁺ base contacts are concurrently formed by thermal anneal (e.g., at800° C. to 950° C.). Drying/curing and emitter anneal may both beperformed sequentially in a diffusion or in-line belt-driven furnace. Inone embodiment, the 3-D TFSC substrate, or stacks of 3-D TFSC substratesin face-to-face contact, may be annealed while placed in a diffusionfurnace, in order to facilitate gas-phase doping formation of n⁺selective emitter junctions. The rear cap dielectric (e.g., oxide) layerprevents boron counter doping of selective emitter prism sidewalls. Step1618 (surface passivation and ARC) corresponds to step 1588 in FIG. 155;step 1620 (metal coat) corresponds to step 1590; step 1622 (firethrough) corresponds to steps 1592; step 1624 (FGA) corresponds to step1594; step 1626 (plating) corresponds to step 1596; step 1628 (mounting)corresponds to step 1598; and step 1630 (proceeding with packaging)corresponds to step 1600.

FIGS. 157 through 159 show additional alternative process flowembodiments 1640, 1670, and 1700 for fabricating dual-aperturehexagonal-prism 3-D TFSCs without rear base layers. While the processflow embodiments of FIGS. 154 to 156 use a fire-through metallizationprocess to form the emitter and base contact metallization regions (andin FIGS. 154 and 155 also to form the p⁺⁺ heavily doped base contactregions), the process flow embodiments of FIGS. 157 to 159 employselective plating (electroplating and/or electroless plating and/orgalvanic plating) to selectively form the emitter and base contactmetallization regions. Thus, the process flows outlined in FIGS. 157 to159 eliminate the need for a medium or high-temperature (e.g., 700° C.to 850° C.) fire-through metallization process and can result inimproved metallization contacts with reduced dark current and junctionleakage.

FIG. 157 shows an alternative process flow 1640. Step 1642 (providing asubstrate) corresponds to step 1612 of FIG. 156; step 1644 (selectivecoating) corresponds to step 1614; step 1646 (self-aligned selectiveemitter) corresponds to step 1616; and step 1648 (surface passivationand ARC) corresponds to step 1618. Step 1650 involves self-alignedmetallization etching. The top and rear portions of the honeycomb prismsare selectively coated (to heights equal to or less than the n-type andp-type dopant source layers, respectively) with dielectric etchantliquid or paste layers. This dielectric etchant coating may be done byself-aligned single-sided or double-sided roller coating usingpaste/liquid sources, liquid-dip coating by dipping in a specifiedliquid etchant source depth, ink-jet coating, or spray coating. Thisetching step strips, to controlled heights, dielectric layer or layerstack (e.g., oxide or oxide/nitride) from top and rear portions ofhoneycomb prisms to expose silicon. Next, the TFSC substrate is rinsedand subsequently dried. Step 1652 involves self-aligned metallization(using electroless plating, galvanic plating, or electroplating). Theexposed front and rear emitter and base metallized regions areconcurrently formed using selective electroplating and/or electrolessplating to form a single layer or multiple layers of high-conductivitymetallized regions. Embodiments include silver, aluminum, nickel,titanium, cobalt, or tantalum. For instance, the plated metal stack mayinclude a thin (50 to 500 nanometers) barrier and adhesion layer such asnickel (nickel) followed by a relatively thick (2 to 15 microns) layerof high-conductivity metal (silver or copper or aluminum). If ahigh-conductivity metal other than silver is used for the thickmetallization layer, a final flash coat of silver may be used to createa high-reflectivity surface coating in order to improve light reflectionand trapping into the 3-D cells (by the emitter metallization contact).Step 1654 (optional FGA) corresponds to step 1624 of FIG. 156; step 1656(optional plating) corresponds to step 1626; step 1658 (mounting)corresponds to step 1628; and step 1660 (proceeding with module assemblyand packaging) corresponds to step 1630.

FIG. 159 shows an alternative process flow 1670. Step 1672 (providing asubstrate) corresponds to step 1642 of FIG. 157; step 1674 (selectivecoating) corresponds to step 1644; and step 1676 (self-aligned selectiveemitter) corresponds to step 1646. Step 1678 involves surfacepassivation thermal oxidation. A thermal oxide layer is grown, in oneembodiment by steam oxidation. In one embodiment, this layer is between5 and 300 nanometers (preferably 10 to 50 nanometers), with theoxidation performed at 800° C. to 950° C. (lower temperatures andthinner oxides preferred). Alternatively, this step may be merged intothe prior diffusion step in the diffusion furnace, to be performedsequentially in a multi-zone in-line diffusion furnace after theselective emitter and base diffusion step. In one embodiment, thecombined thermal budget associated with the diffusion and thermaloxidation steps results in 3-D honeycomb-prism selective emitter andheavily-doped emitter contact sheet resistance values of 80-150 Ω/squareand 10-70 Ω/square, respectively. Step 1680 involves self-alignedmetallization (etching). The 3-D TFSC substrate is covered with thecured dopant source layers on top (emitter side) and rear (base side)and an undoped dielectric cap (sealant such as cured oxide) layer onrear. The top and rear cured dielectric and solid dopant source portionsare selectively etched using a suitable dielectric etchant (e.g., anHF-based etchant) with relatively high selectivity with respect tothermal oxide. This selectively strips the cured dopant source segmentsand undoped capping (oxide) dielectric on the top and rear portions ofthe honeycomb prisms and exposes silicon in those regions, whileremoving only a small fraction of thermal oxide from honeycomb-prismsidewalls. Therefore, the honeycomb-prism sidewalls corresponding to theselective emitter regions and outside the emitter and base contactregions remain covered with thermal oxide. Step 1682 (plating)corresponds to step 1652 of FIG. 157; step 1684 (FGA) corresponds tostep 1654; step 1686 (plating) corresponds to step 1656; and step 1688(mounting) corresponds to step 1658. Step 1690 involves an optional stepof depositing a passivation and anti-reflection coating (ARC) layer(e.g., 50-200 nanometers of PVD or PECVD SiN_(x)) layer on mountedcells. Step 1692 (proceeding with packaging) corresponds to step 1660 ofFIG. 157.

FIG. 159 shows an alternative process flow 1700. Step 1702 (providing asubstrate) corresponds to step 1672 of FIG. 158. Step 1704 involvesselectively coating the top and rear portions of the 3-D substrate (inone embodiment, the top and rear 2 to 10 microns in height) with aphosphorus (i.e., n-type dopant) liquid/paste source on top and anundoped capping (sealant) dielectric such as oxide liquid/paste sourceon rear. This coating may be done by self-aligned single-sided or2-sided roller coating using liquid/paste sources, liquid-dip coating bydipping in a controlled liquid source depth, ink-jet coating, or spraycoating. Next, the dopant and capping dielectric (oxide) layers aredried and cured (e.g., using thermal curing at 250° C. to 400° C. or byUV irradiation). Step 1706 (self-aligned selective emitter) correspondsto step 1676 of FIG. 158. Step 1708 (surface passivation and ARC)corresponds to step 1648 of FIG. 157. Step 1710 involves selectivelyetching the top and rear portions of the cells in preparation forself-aligned metallization The top and rear portions of the honeycombprisms (to heights equal to or less than the n-type and p-type dopantsource layers, respectively) are selectively coated with dielectricetchant liquid or paste layers. This coating may be done by self-alignedsingle-sided or double-sided roller coating using paste/liquid sources,liquid-dip coating by dipping in a specified liquid etchant sourcedepth, ink-jet coating, or spray coating) to strip controlled heights ofoxide/nitride dielectrics from top and rear portions of honeycomb prismsto expose silicon at the honeycomb ridges. Next, the TFSC substrate isrinsed and dried. Step 1712 involves self-aligned metallization (in oneembodiment, using plating). The rear base aluminum metallized regionsare formed by selective electroplating or electroless plating. Thisaluminum plating process may be limited to the rear base regions. Step1714 involves self-aligned metallization (anneal). The substrate goesthrough an annealing or firing process to form rear aluminum-doped p⁺base contacts. Then, an optional forming gas anneal (FGA) is performed(e.g., 350° C. to 450° C.) to reduce interconnect resistance and helpwith surface/bulk passivation. Step 1716 involves self-alignedmetallization (plating). A high-conductivity metal (such as silver orcopper) (e.g., 2 to 12 microns) is selectively/concurrently deposited onthe top honeycomb ridges (emitter) and rear honeycomb ridges (aluminummetallized base contacts) by plating. If necessary, a suitable thin(e.g., 50 to 200 nanometers) refractory metal barrier layer such asnickel is first selectively deposited by plating (in one embodiment,electroless plating) before depositing the silver or copper layer. Next,if necessary, the TFSC substrate may be flash coated with a thin layerof high-reflectivity silver (not needed if the thick metallization layeris also silver). Alternatively, this may be merged into the priorplating step. Step 1718 (mounting) corresponds to step 1688 of FIG. 158;step 1720 (proceeding with packaging) corresponds to step 1692.

The above process steps may be performed on integrated belt-drivenprocess equipment, as shown in FIGS. 149 through 151 above for 3-Dsingle aperture TFSC designs.

The following section describes the use of 3-D dual aperture thin-filmTFSC substrates in 3-D TFSCs.

FIG. 160A shows a Y-Y cross-sectional view 1740 of a self-supportinghexagonal-prism dual-aperture 3-D thin-film TFSC substrate with a thinsilicon frame 1742 and without a rear base layer, before TFSCfabrication. In this embodiment, the thin silicon frame 1742 issquare-shaped with the same thickness as 3-D thin-film TFSC substrate,with a silicon frame width 114 of 50 to 250 microns. The final 3-Dthin-film TFSC substrate is may be square-shaped, with dimensionsranging from roughly 125 mm×125 mm to greater than 200 mm×200 mm.

FIG. 160B shows a view 1750 of the substrate of FIG. 160A after TFSCfabrication, indicating the emitter and base metal contacts for thehexagonal-prism 3-D TFSC without a thick silicon frame. The thin siliconframe also shows the wrap-around (may be made wrap through instead)emitter contact metal for ease of module assembly (making the cellemitter and base contacts to the module from the cell backside). Thethin silicon frame extension may be made during the 3-D substratefabrication process. For example, the thin frame may be made by thesilicon deposition process as the hexagonal-prism structure is formed inthe template. A self-aligned peripheral frontside contact 1752 isconnected to the 3-D TFSC hexagonal frontside emitter contact at theframe edge (connected to the distributed self-aligned emitter contact1754). The self-aligned emitter contact 1754 may be formed by rollercoating, liquid-dip coating, or spray-jet-coating and fired through thepassivation layer (nitride or oxide/nitride stack). Alternatively, theself-aligned emitter contact 1754 may be formed using a selectiveplating process. A self-aligned wrap-around emitter contact is on thethin silicon frame (the wrap-around contact may be replaced with awrap-through contact). Both frontside and backside contacts areaccessible on the rear side of the cell for automated module assembly.

FIG. 161A shows a Y-Y cross-sectional view 1760 of a self-supportinghexagonal-prism dual-aperture 3-D thin-film TFSC substrate with a thickperipheral silicon frame 1762. FIG. 161B shows a view 1770 of thesubstrate of FIG. 161A after cell fabrication, indicating the emitterand base metal contact metals. The thick-Si frame also shows theself-aligned wrap-around emitter contact metal 1752 (on thick siliconframe 1762) for ease of module assembly (the wrap-around contact may bereplaced with a wrap-through contact). The thick silicon frame may beseparately made of low-cost silicon wafers (e.g., laser cut from wafers)and then attached to the hexagonal prism 3-D TFSC substrate by asuitable technique. For example attachment may occur during theepitaxial silicon deposition process, to seal the frame to thehexagonal-prism structure, or by electron-beam welding, etc. Thehexagonal-prism 3-D TFSC fabrication process flows of this disclosureutilize roller coating, spray coating or liquid-dip coating followed bythermal or UV curing for self-aligned formation of solid dopant layersand metal layers (the latter for the cell fabrication process flowembodiments using fire-through metallization). In one of the processflow embodiments, a roller coating (or spray coating or liquid-dipcoating) process may also be used for controlled dielectric etching fromhexagonal ridges. Also shown are the self-aligned backside hexagonalbase contacts 1772 (roller-coated, dip-ink-coated, or spray-jet-coatedand fired through the passivation layer such as nitride or oxide/nitridestack). Both frontside emitter metallization contact and backside basecontact are accessible on the rear side of the cell for automated moduleassembly.

FIGS. 162A to 166 show various representative dual-aperturehexagonal-prism 3-D TFSC structures (with tapered prism sidewalls). Oneembodiment uses crystalline silicon (c-Si) for the TFSCs; polysilicon,amorphous silicon, as well as non-Si absorbers may also be used inalternative embodiments. These TFSCs are fabricated based on the processembodiments described earlier in FIGS. 154-159. These cross-sectionalviews correspond to dual-aperture hexagonal-prism 3-D TFSCs without rearbase layers. The hexagonal prism 3-D TFSCs shown in these FIGURES havetapered prism sidewalls (in one embodiment, narrower emitter siliconwidth on the top and wider base silicon width at the bottom).

FIG. 162A shows a Y-Y cross-sectional view 1780 of a single unit cellwithin a dual-aperture hexagonal-prism 3-D TFSC without a rear baselayer, with tapered prism posts 1342. The central region 1782 of theprism post 1342 contains p-type silicon serving as the base region. Thecell view 1780 is after self-aligned formation of: heavily-doped emittercontact diffusion regions 1784 (e.g., more heavily-doped withphosphorus, n⁺⁺ doped emitter contact regions on the hexagonal prism tophexagonal ridges as shown); selective emitter regions 1786 (e.g., lessheavily-doped with phosphorus, n⁺ selective emitter on thehexagonal-prism sidewall surfaces as shown); and heavily-doped basecontact regions 1788 (e.g., more heavily-doped with boron, p⁺⁺ dopedbase contact regions on the hexagonal prism rear hexagonal ridges asshown). The hexagonal-prism sidewalls are in-situ-doped (at the time of3-D TFSC substrate fabrication) with boron (either uniformly or in agraded profile, more lightly doped at the prism sidewall surface andmore heavily doped towards the sidewall vertical center axis). The cellview 1780 is before formation of emitter and base contact metallization.Further, each sidewall 1342 has a top silicon thickness (T_(st)) 426 andbottom silicon thickness (T_(sb)) 430. Each sidewall 1342 has anoff-normal taper sidewall angle, θ 432.

Compared to the doping polarities shown in FIG. 162A, all the celldoping polarities may be inverted. FIG. 162B shows a view 1790 of thesame TFSC as in FIG. 162A but with inverted doping polarities (i.e.,n-type base and p-type emitter regions).

FIG. 162B shows a Y-Y cross-sectional view 1790 of a single unit cellwithin a dual-aperture hexagonal-prism 3-D TFSC without a rear baselayer, with tapered prism posts 1342. The central region 1792 of theprism post 1342 contains n-type silicon serving as the cell base region.The cell view 1790 is after self-aligned formation of: heavily-doped p⁺⁺emitter contact regions 1794; selective p⁺ selective emitter regions1352; and heavily-doped n⁺⁺ base contact regions 1796. The cell view1790 is before formation of emitter and base contact metallization.

Continuing from FIG. 162A, FIG. 163A shows a Y-Y cross-sectional view1800 of a unit cell within a hexagonal prism 3-D TFSC, with p-type prismposts 1342. The cell view 1800 is after self-aligned formation of:surface passivation oxide and anti-reflection coating (ARC) layer(s)1802; emitter contact metal 1354 (e.g., silver, copper, aluminum; usinga refractory metal barrier if necessary) with coverage height L_(em)1356 and base contact metal 1804 (silver, copper, aluminum; using arefractory metal barrier if necessary) with coverage height L_(bm) 1806by fire-through and/or selective plating. The hexagonal-prism sidewallsare in-situ-doped (at the time of 3-D TFSC substrate fabrication) withboron (either uniformly or in a graded profile, more lightly doped atthe prism sidewall surface and more heavily doped towards the sidewallvertical center axis). The cell view is before mounting the cells onto ahighly reflective rear mirror.

As noted above, all the cell doping polarities may be inverted.Continuing from FIG. 162B, FIG. 163B shows a Y-Y cross-sectional view1810 of a unit cell within a dual-aperture hexagonal-prism 3-D TFSC,with n-type prism posts 1792. All the cell doping polarities areopposite of those shown in FIG. 163A. The cell view 1810 is afterself-aligned formation of: surface passivation oxide and anti-reflectioncoating (ARC) layer(s) 1802; emitter contact metal 1354 (silver, copper,aluminum; using a refractory metal barrier if necessary) with coverageheight L_(em) 1356 and base contact metal 1804 (silver, copper,aluminum; using a refractory metal barrier if necessary) with coverageheight L_(bm) 1806 by fire-through and/or selective plating. Thehexagonal-prism sidewalls are in-situ-doped (at the time of 3-D TFSCsubstrate fabrication) with phosphorus (either uniformly or in a gradedprofile, more lightly doped at the prism sidewall surface and moreheavily doped towards the sidewall vertical center axis). The cell viewis before mounting the cells onto a highly reflective rear mirror.

Continuing from FIG. 163A, FIG. 164A shows a Y-Y cross-sectional view1820 of a unit cell after mounting the cell onto a detached highlyreflective rear specular mirror 1336 with a polished mirror surface. Adetached mirror is not integrated through direct material deposition onthe cell. In one embodiment, the mirror is made of silver and/oraluminum (or a polished disk coated with silver and/or aluminum and/orAu), with reflectance greater than 95% for λ between 800 and 1200nanometers. The rear mirror surface to base contact spacing (S) 1434 maybe in the range of 0 (i.e., direct contact between the rear mirror andthe hexagonal base contact; in this case the rear mirror also serves asthe base electrical interconnect plane) up to roughly H (or a spacing asmuch as the height of the hexagonal prism cell—up to 100 to 500microns).

Continuing from FIG. 163B, FIG. 164B shows a Y-Y cross-sectional view1830 of a unit cell after mounting the cell onto a highly reflectiverear diffuse mirror 1432 with textured mirror surface. In oneembodiment, the mirror is silver-coated, with reflectivity greater than95% for λ between 800 and 1200 nanometers.

FIG. 165A shows a cross-sectional view 1840 of several prism unit cellsfrom the hexagonal-prism TFSC shown in FIG. 164A.

FIG. 165B shows a cross-sectional view 1850 of several prism unit cellsfrom the hexagonal-prism TFSC shown in FIG. 164B.

FIG. 166 shows a cross-sectional view 1860 of the hexagonal prism TFSCabove, mounted on and in contact with a detached diffuse (rough)high-reflectivity rear mirror 1432 (made of silver and/or aluminumand/or Au or their coatings). The rear mirror surface to base contactspacing (S) depicted here is zero and, thus, there is direct contactbetween the rear diffuse mirror and the cell base contact metal.Therefore, the rear mirror also serves as the base electricalinterconnect plane. The rear mirror may be part of the solar moduleassembly (e.g., a printed-circuit board with silver-coated copperinterconnects connecting to the cell emitter and base contactmetallization and connecting the cells in series). This FIGURE showsseveral prism unit cells and corresponds to the same hexagonal-prismTFSC shown in FIG. 164B but with a zero mirror-to-base contact spacing1434 (FIG. 164B shows one unit cell).

FIGS. 162A to 166 show 3-D TFSCs of this disclosure with tapered prismsidewalls. It is also possible to fabricate hexagonal-prism 3-D TFSCs(with or without rear base layers) which have substantially verticalprism sidewalls. For example, FIG. 167A shows a Y-Y cross-sectional view1870 of a unit cell. Except for the sidewall profile (being vertical),other cell design features are essentially similar to those of thetapered-wall cell shown in FIG. 162A. Note the uniform hexagonal wallthickness (T_(s)) 1872 compared to FIG. 162A.

Continuing from FIG. 167A, FIG. 167B shows a Y-Y cross-sectional view1880 of a unit cell. Except for the sidewall profile (being vertical),other cell features are essentially similar to those of the tapered-wallcell shown in FIG. 162A. Note L_(tm) 1882 compared to FIG. 162A.

Continuing from FIG. 167B, FIG. 168 shows a Y-Y cross-sectional view1890 of a unit cell after mounting the cell onto a reflective specularrear mirror 1336. This hexagonal prism cell has vertical prism sidewalls(i.e., equal top emitter and rear base prism sidewall widths). Exceptfor the sidewall profile (being vertical), other cell features areessentially similar to those of the tapered-wall cell shown in FIG.164A. Again, note L_(tm) 1882 and the uniform hexagonal wall thickness(T_(s)) 1872.

Again continuing from FIG. 167B, FIG. 169 shows a Y-Y cross-sectionalview 1900 of a unit cell after mounting the cell onto a diffuse rearmirror 1432. This hexagonal prism cell has vertical prism sidewalls(i.e., equal top emitter and rear base prism sidewall widths). Exceptfor the sidewall profile (being vertical), other cell features aresimilar to those of the tapered-wall cell shown in FIG. 164B. Again,note L_(tm) 1882 and the uniform hexagonal wall thickness (T_(s)) 1872.

FIG. 170 shows a Y-Y cross-sectional view 1910 of a dual-aperturehexagonal-prism 3-D TFSC with vertical prism sidewalls and without aflat base layer, mounted on a detached specular high-reflectivity rearmirror 1336 (made of silver and/or aluminum and/or Au or theircoatings). The rear mirror surface to base contact spacing (S) 1434 maybe in the range of 0 (i.e., direct contact between the rear mirror andthe base contact; in this case the rear mirror also serves as the baseinterconnect plane) up to roughly H (or a spacing as much as the heightof the hexagonal prism cell). This FIGURE shows several prism unit cellsand corresponds to the same hexagonal-prism cell shown in FIG. 168(which shows one unit cell).

FIG. 171 shows a Y-Y cross-sectional view 1920 of a dual-aperturehexagonal-prism 3-D TFSC with vertical prism sidewalls and without aflat base layer, mounted on a detached diffuse high-reflectivity rearmirror 1432 (made of silver and/or aluminum and/or Au or theircoatings). The rear mirror surface to base contact spacing (S) 1434 maybe in the range of zero (i.e., direct contact between the rear mirrorand the base contact; in this case the rear mirror also serves as thebase interconnect plane) up to roughly H (or a spacing as much as theheight of the hexagonal prism cell). This FIGURE shows several prismunit cells and corresponds to the same hexagonal prism cell shown inFIG. 169 (which shows one unit cell).

FIGS. 172A and 172B show enlarged views of a hexagonal-prism unit cellin a hexagonal-prism 3-D TFSC of this disclosure, both before and aftercell fabrication (shown before and after self-aligned cellmetallization).

FIG. 172A shows a quasi 3-D view 1930 of a single unit cell from aregular dual-aperture hexagonal-prism TFSC of this disclosure (shown forthe cell without a rear base layer), before self-aligned base andemitter contact metallization. The prism sidewall surfaces are doped toform the selective emitter junctions (e.g., n⁺p junctions in boron-dopedsilicon base). This FIGURE corresponds to the cell fabrication processflow embodiments in FIGS. 154-159. FIG. 172A shows top hexagonal opening1104, which may form the frontside self-aligned emitter metallizationcontacts; and rear (bottom) hexagonal opening 1106, which may form therear selective base self-aligned contacts.

FIG. 172B shows a quasi 3-D view 1940 of a single unit cell from aregular hexagonal prism TFSC of this disclosure, after cell fabrication,including self-aligned base and emitter contact metallization. The darkregion on the top 1332 of the unit cell is the self-aligned emittercontact metal; the rear 1334 of the unit cell is the self-aligned basecontact metal. The prism sidewall surfaces are doped to form theselective emitter junctions (e.g., shallow n⁺p junctions with a junctiondepth of 0.2 to 0.5 micron in boron-doped silicon base).

FIG. 173 shows a partial view 1950 of multiple adjacent prism unit cells(compared to a single unit cell in FIG. 172B) after mounting the cellrear base side (base contact metal regions 1334) onto a rear mirror 1952(specular 1336 or diffuse rear mirror 1432, as shown in previousFIGURES). The rear mirror 1952 may be made of a suitable planar material(e.g., copper or aluminum) flash-coated with silver or aluminum (e.g.,using plating). The rear mirror may be copper interconnect pads orislands (plated with silver and/or aluminum) on a printed-circuit board(PCB) within a solar module assembly. Thus, the rear mirrors may alsoserve as base electrical interconnect planes. The rear base contactmetal regions 1334 may be soldered or glued (or simply placed) onto theplated copper pads on the solar module PCB assembly. There is anoptional step of silver flash coating of the assembled module structureto coat the solder joint surfaces with optically reflective silver(silver).

One of the important features of the hexagonal-prism 3-D TFSCs of thisdisclosure (both with and without rear base layers) is highly efficientcollection of photogenerated carriers due to the unique devicestructure. For instance, the selective emitter junctions cover bothsurfaces of the prism sidewalls. Therefore, in the case ofhexagonal-prism 3-D TFSCs with n⁺ selective emitter junctions andin-situ-boron-doped starting hexagonal-prism 3-D thin-film TFSCsubstrates, any photogenerated electrons in the prism sidewalls areefficiently collected by the selective emitter junctions that cover bothsurfaces of the prism sidewalls. This makes the cells of this disclosureless demanding in terms of substrate defects or minority carrierdiffusion length.

A minority carrier diffusion length just a few times larger than theprism sidewall thickness (e.g., by a factor of 2 to 5) providessufficient material quality for effective collection of thephotogenerated carriers with negligible recombination losses by one ofthe two selective emitter junctions located nearby in the adjacentsidewall surfaces. For instance, for a prism sidewall silicon filmthickness of 5 microns, a minority carrier diffusion length of greaterthan 10 to 15 microns should provide sufficient substrate lifetimequality for achieving ultra-high cell efficiency due to minimalrecombination losses. This is a key advantage compared to the currentsilicon wafer solar cells, where the photogenerated carriers may have totravel as far as 150 to 200 microns to be collected (thus, requiring aminority carrier diffusion length of at least hundreds of micrometers).As a result of this key advantage, it is possible to fabricatehigh-performance hexagonal-prism 3-D TFSCs based on the embodiments anddesigns of this disclosure (and their many derivatives) using apolycrystalline semiconductor absorber such as multicrystalline siliconor polysilicon, instead of the more expensive monocrystalline silicon.

FIG. 174 shows a view 2200 of a single unit cell 116 from thehexagonal-prism 3-D thin-film TFSC substrate for reference withcalculations of the unit cell prism sidewall area, hexagonal-prism unitcell aperture area, and the sidewall to planar base area ratio. Thesecalculations are performed for both types of single-aperture anddual-aperture hexagonal-prism 3-D TFSC substrates, both with and withoutrear base layers. The long diagonal dimension of the unit cell hexagon(d) 1124 may be chosen in the range of roughly 50 microns to roughly 500microns, with more typical values in the range of 100 to 250 microns.The hexagonal-prism aspect ratio (H/d or height 1132 to long hexagonaldiagonal dimension 1124 ratio) may be anywhere between roughly 0.5 and5, with more typical values between 1 and 3. The silicon film thicknessW 118 may be in the range of roughly 1 to 30 microns, with typicalvalues in the range of 2 to 10 microns. Point A 2202 is the mid-pointbetween H₅ 1120 and H₆ 1122. Point B 2204 is the mid-point between H₂1114 and H₃ 1116.

Short hexagonal diagonal distance between opposite unit cell points 2202and 2204 (h)

h=(√3/2)d=0.866d

Planar surface Area of hexagonal-prism 3-D TFSC substrate hexagonal top(or rear) base (S_(hb))

S _(hb)=[(3√3)/8]·d ²

Surface Area of hexagonal-prism unit cell cavity walls (fordual-aperture cell without rear flat silicon layer) (S_(hp))

S _(hp)=3·d·H

Surface Area of hexagonal-prism unit cell cavity walls (for cell withrear flat silicon layer) (S_(hp))

S _(hp)=3·d·H+[(3√3)/8]·d ²

For dual-aperture hexagonal-prism TFSC substrate without rear flatsilicon layer:

S _(hp) /S _(hb)=[8/√3]·(H/d)

For single-aperture hexagonal-prism TFSC substrate with rear flatsilicon layer:

S _(hp) /S _(hb)=[8/√3]·(H/d)+1

For the following calculations, assume W is the hexagonal prism sidewallsilicon film thickness. Also assume that W/2 is the prism base siliconthickness for hexagonal-prism TFSC substrate with rear flat siliconlayer:

Volume of silicon per prism unit cell for dual-aperture hexagonal-prismwithout rear flat silicon layer is:

V _(hp)=3·d·H·(W/2)=(3/2)d·H·W

Volume of silicon per prism unit cell for single-aperturehexagonal-prism with rear flat silicon layer is:

V _(hp)=(3/2)d·H·W+[(3√3)/8]·d ²(W/2)

The ratio of silicon volume V_(hp)/V_(F) (or mass M_(hp)/M_(F)) for thehexagonal-prism 3-D thin-film TFSC substrate with prism sidewallthickness of W to volume (or mass) for a flat silicon wafer or film withthe wafer thickness W_(F) is as follows (shown for dual-aperturehoneycomb-prism TFSC substrate without rear flat silicon layer):

V _(hp) /V _(F) =M _(hp) /M _(F)=(4/√3)[(H/d)·(W/W _(F))]

The ratio for the single-aperture honeycomb-prism TFSC substrate withthe rear flat silicon layer is as follows:

V _(hp) /V _(F) =M _(hp) /M _(F)=(4/√3)[(H/d)·(W/W _(F))]+[W/(2·W _(F))]

FIGS. 175 and 176 help to illustrate the important results of the abovecalculations. FIG. 175 shows a graph of hexagonal-prism area ratio, forTFSC substrates with and without rear base layers. The Y-axis is theratio of hexagonal-prism unit cell aperture area to hexagonal-prism unitcell base area. The X-axis is the ratio of hexagonal-prism unit cellheight to unit cell base diagonal dimension (H/d or unit cell aspectratio). FIG. 176 shows a graph of hexagonal-prism mass ratio, for 3-DTFSC substrates with and without rear base layers. The Y-axis is theratio of honeycomb-prism substrate silicon mass to the mass of a flatsilicon wafer. The X-axis is the ratio of honeycomb prism sidewallthickness to the thickness of a flat silicon wafer.

FIGS. 175 and 176 show that the hexagonal-prism 3-D TFSC substrates maybe designed such that they have much larger solar absorption surfacearea (in conjunction with a 3-D structure facilitating lightcapture/trapping) while consuming much less silicon volume (and hencemuch less silicon mass) compared to standard flat silicon wafers.

For a hexagonal-prism TFSC substrate without a rear flat layer, thetotal unit cell absorber silicon volume (or mass) is half the volume (ormass) of the unit cell prism sidewalls. This is because only half of thesidewall volume or mass belongs to each unit cell. For a hexagonal-prismTFSC substrate with a rear flat layer, the total unit cell absorbervolume (or mass) includes the sum of half the volume (or mass) of theunit cell prism sidewalls and the volume (or mass) of the rear baselayer.

As indicated, the 3-D thin-film hexagonal-prism TFSC substrate designsof this disclosure have substantially smaller silicon volume (or mass)compared to the current state-of-the-art solar cell silicon wafers,which typically use wafers with thicknesses of more than 200 microns.This is particularly true for designs with unit cell aspect ratios (H/d)of less than 3. For a given thin silicon film thickness and substratesize, the amount of silicon material used (as measured by the totalsilicon surface area, volume, or mass) in the hexagonal prism 3-D TFSCsubstrate is larger than that of a co-planar (flat) substrate with thesame dimensions. However, the amount of silicon used is considerablyless than the amount of silicon used in standard crystalline silicon(c-Si) wafer solar cells. The hexagonal-prism 3-D TFSCs of the currentdisclosure consume 3 times to over 10 times less silicon than standardc-Si wafer solar cells.

To achieve efficient light capture/trapping within the 3-D thin-filmhexagonal-prism cell structure and very low effective surfacereflectance with a reasonable (i.e., not excessive) area enlargementfactor of S_(hp)/S_(hb), the prism aspect ratio H/d may be in the rangeof 1 to 3.

One of the major advantages of the hexagonal prism 3-D TFSCs of thisdisclosure is negligible shadowing (optical reflectance loss) effects ofthe emitter and base contact metallization. Essentially any reflectionsfrom the base hexagonal contact metal may be received by the hexagonalprism silicon absorber and may contribute to the photogenerated current.Also, the combination of the thin prism sidewall on the top emitter sidein conjunction with the rounding of the top hexagonal ridges beforeemitter contact metallization and the unit folded structure of theemitter contact metallization may ensure that most of the sunlightreflections would be redirected into the prism cavity sidewalls and/orthe hexagonal prism cavity base layer (the rear base silicon layer inthe case of cells with rear base layers and also the rear mirror, in thecase of all hexagonal prism cell designs with and without rear baselayers). A small fraction of the sunlight incident on the top of thereflective emitter contact may escape the hexagonal prism cellcavity/aperture. This may be further minimized by coating the cell withan additional layer of mechanical protection and ARC layer duringglassless module assembly. As an example, FIG. 177 shows a schematicdiagram 2210 of ray tracing for solar rays 2212 incident on a hexagonalprism unit cell reflective emitter contact. As indicated here, most ofthe sunlight rays (including incident rays R₁ 2214, R₂ 2216, R₃ 2218, R₄2220, R₅ 2222) incident on the folded emitter contact are reflected back(shown as reflected rays R₁ 2215, R₂ 2217, R₃ 2219, R₄ 2221, R₅ 2223,with R₁ 2225 being the lone exception) into the prism unit cell cavity2224 and/or onto the rear mirror 2226 (or the rear base layer in thecase of cells with rear base layers) and ultimately contribute to theTFSC electricity generation (due to the folded emitter contact metaldesign on the hexagonal prism top ridges). The prism sidewall and theemitter contact metal on the top may be optimally tapered in order tominimize any shadowing losses due to the emitter contact metal.Moreover, a protective transparent coating layer formed over the cellduring the final solar module assembly (e.g., a proper coating layerformed by liquid spraying and curing over the cell) may further reducethe reflection losses associated with the top emitter contact metal.Optical waveguiding of the rays incident on the top of the emittercontact metal redirects the incident rays from the top of the emittercontact metal into the hexagonal-prism unit cell cavity 2224.

FIG. 178 shows ten rays 2212 incident on a hexagonal-prism unit cell ata normal angle of incidence for the purpose of ray tracing calculations.FIG. 179 shows ten rays 2212 at an angle of incidence of 45 degrees forthe purpose of ray tracing calculations. FIG. 180 shows three rays 2212at a normal angle of incidence for the purpose of ray tracingcalculations. FIG. 181 shows three rays 2212 at an angle of incidence of45 degrees for the purpose of ray tracing calculations.

FIG. 182 shows a summary graph of short circuit current density versusangle of incidence for several embodiments of the solar cells of thecurrent disclosure. FIG. 182 also shows the ray tracing results for ahexagonal-prism unit cell with a base layer, indicating super-efficientlight trapping. FIG. 183 shows a summary graph of the basic ray tracingmodeling results for a single-aperture hexagonal-prism cell design ofthis disclosure, indicating both the STC cell efficiency andshort-circuit current versus unit cell prism height. Note thatultra-high-efficiency (greater than 25% STC efficiency) capability ofthe cell designs of this invention has been verified through modeling.FIG. 184 shows a summary graph of the results of ray tracing modelingresults in a single-aperture hexagonal-prism cell design of thisinvention (with a base layer), with maximum photocurrent plotted againstincident angle. This FIGURE indicates that the emitter wrap-aroundmetallization actually increases the photogenerated current density as aresult of improved light trapping within the prism sidewalls.

Throughout descriptions of various embodiments of this disclosure, ithas been stated that the hexagonal-prism 3-D TFSC substrate may be dopedin-situ during semiconductor layer deposition (e.g., epitaxial silicongrowth) onto the reusable templates. The hexagonal-prism 3-D TFSCsubstrate base doping may be uniform or graded. In case of optimalgrading of the hexagonal prism 3-D TFSC substrate doping, the substratemay provide an internal electric field due to the graded substratedoping which may facilitate or aid photogenerated carrier collection dueto a field-assisted carrier drift component. Dopant concentrationgrading may be done linearly, logarithmically, or in another suitablescaling method. As an example, FIG. 185 shows a graph of the selectiveemitter phosphorus and 3-D substrate boron doping profiles (prismsidewall-to-sidewall doping profile) in hexagonal prism 3-D TFSCs ofthis disclosure, indicating a representative graded base doping profile.As indicated before, the graded base doping profile helps withphotogenerated carrier collection efficiency and a reduction of the baseparasitic resistance (and the resulting ohmic losses), thus, improvingthe short circuit current and fill factor of the cells. This example isshown for a boron-doped base and phosphorus-doped emitter. In the caseof hexagonal-prism cells with rear base layers, the graded base dopingalso creates a graded boron doping profile with the boron concentrationbeing lower on the top surface (emitter side) of the rear base layer andincreasing towards the lower surface (base side) of the rear base layer.This provides a desirable back-surface field (BSF) effect which improvesthe carrier collection efficiency in the rear base layer as well andreduces the surface recombination velocity in the rear base layer.

In order to maximize the hexagonal-prism 3-D TFSC efficiency, it isimportant to design the cell such that the ohmic losses associated withthe electron and hole currents within the cell structure are minimized.This is in addition to the need to make the emitter and baseinterconnect metallization ohmic losses negligible. Assuming aboron-doped base and an n⁺ selective emitter, the ohmic losses withinthe hexagonal-prism cell structure are dominated by theemitter-collected electrons traveling along the selective emitter regiontowards the emitter contact metal on the prism top (or the emittercurrent traveling from the emitter contact metal towards the selectiveemitter) as well as the base-collected holes (or the base current)traveling along the hexagonal prism boron-doped sidewall bulk towardsthe base contact metal at the rear of the cell (the above-mentionedcarrier polarities would be inverted for phosphorus-doped n-type baseand p⁺ selective emitter).

Referring to FIG. 186, the following section outlines the summarycalculation of the hexagonal prism TFSC ohmic losses due to the basecurrent along the prism sidewalls. These calculations assume that allphotogenerated current is in the prism sidewalls (and assume that thephotogenerated current is negligible in the rear base layer in the cellswith rear base layers). The results should be fairly similar to thoseobtained even after taking into account the photogenerated current dueto the rear base layer for the cells with rear base layers. It is alsoassumed that the photogenerated current contribution is uniform alongthe height of the prism sidewall (i.e., each fixed small verticalsegment of the prism sidewall contributes equally to the photogeneratedcurrent. Thus, the base ohmic losses may be estimated through a simpleintegration of the differential ohmic losses along the prism sidewallfrom rear to top of the prism (or from top to rear of the prism) as thephotogenerated current increases from 0 to the maximum hexagonal-prismunit cell current. For the base ohmic losses to be negligible (less than0.1% or 1/1000 of the maximum cell power, assuming 200 W/m² maximum cellpower generation), it is determined that the prism sidewall base sheetresistance should be less than roughly 300 Ω/square. This determines theminimum base boron doping concentration. In practice, the sheetresistance is chosen to be roughly 300 Ω/square and not much below thatin order to prevent lowering of the minority carrier lifetime as aresult of excessive base doping concentration. Minority carrierlifetimes in the range of more than 10 to more than 100 microseconds, orminority carrier diffusion length L_(eff) on the order of a multiple(e.g., by a factor of more than 2 to 5) of the hexagonal prism sidewallthickness should meet the high-efficiency requirements for the cells.

To calculate base ohmic losses, assume all base current is produced inthe prism sidewall. The photo-generated base current is assumed toincrease linearly between z=0 and z=H from 0 to I₀ (where I₀ is thephoto-generated current per prism unit cell). The base current iscollected at the base contact metal at z=H (at the rear of the honeycombprism base layer). The base ohmic loss per unit cell (P_(uc)) iscalculated as follows (R_(sb) is the prism sidewall base sheetresistance; I₀=J₀ S_(hb), where J₀ is the solar cell maximum-powercurrent density and S_(hb) is the surface area of the prism hexagonalaperture).

$P_{uc} = {{\int_{0}^{H}{\frac{2\; R_{sb}{z}}{{3\; }\;}\left( \frac{I_{0}z}{H} \right)^{2}}} = {{\frac{2\; R_{sb}I_{0}^{2}}{9}\left( \frac{H}{d} \right)} = {\frac{2\; R_{sb}J_{0}^{2}S_{hb}^{2}}{9}\left( \frac{H}{d} \right)}}}$

The base ohmic loss per unit area (P_(bl)) is calculated by dividing Pucby the hexagonal aperture area:

$\quad{{\begin{matrix}{P_{bl} = {P_{uc}/S_{hb}}} \\{= {\frac{2\; R_{sb}J_{0}^{2}S_{hb}}{9}\left( \frac{H}{d} \right)}} \\{= {\frac{2\; R_{sb}J_{0}^{2}}{9}{{\left( \frac{H}{d} \right)\left\lbrack {3{\sqrt{3}/8}} \right\rbrack} \cdot d^{2}}}}\end{matrix}P_{bl}} = \frac{\sqrt{3}R_{sb}J_{0}^{2}{dH}}{12}}$

Assume d=100 microns, H=300 microns and J₀=40 mA/cm²:

P _(bl)=(√3/12)R _(sb)(40×10⁻³×10⁴)²(100×10⁻⁶)(300×10⁻⁶)

P _(bl)=6.93×10⁻⁴ R _(sb) (units are in W/m²)

Assume P_(bl) is much less than 200 W/m²; for instance, assume: P_(bl)=(1/1000) 200=0.2 W/m²:

P _(bl)=0.2=6.93×10⁻⁴ R _(sb) →R _(sb)=288.6 Ω/square

Thus, R_(sb) should be less than 300 Ω/square in order for the baseohmic losses to be negligible.

The selective emitter sheet resistance is typically in the range ofroughly 75 Ω/square up to roughly 150 Ω/square. Thus, the ohmic lossesassociated with the selective emitter current are expected to be lessthan the ohmic losses due to the base current (for base sheet resistancevalues on the order of 300 Ω/square as shown above. In summary, we mayconclude that for optimal hexagonal prism 3-D TFSCs of this disclosure,both with and without rear base layers, the overall internal (within Si)ohmic losses due to the base and emitter current components may be madenegligible (or roughly 0.1% of the photogenerated cell power or less),when the cells use prism sidewall layers (and rear base layers in thecase of cells with such rear base layers) which have sufficient basedoping to produce a sheet resistance of roughly 300 Ω/square (or less),and have selective emitter with a sheet resistance value of less than150 Ω/square. Of course, these sheet resistance values may still beincreased while maintaining the internal ohmic losses to much less than1% of the photogenerated cell power.

The hexagonal prism 3-D TFSC maximum base sheet resistance valuecalculated above may be used in conjunction with the silicon thicknessforming the hexagonal prism sidewalls in order to determine the optimal(or near-optimal) base resistivity and doping concentration.

Assuming a boron-doped p-type base, FIG. 187 shows the approximatedesired base boron doping concentration and electrical resistivityvalues for different values of hexagonal prism 3-D silicon filmthickness in the range of 2 to 30 microns in order to keep the baseohmic losses to below 0.1% of the TFSC power. These values were obtainedbased on the calculations outlined above. The same methodology may beapplied to optimizing the cell doping concentrations and profiles forcells using other absorber materials other than c-Si (such aspolysilicon, amorphous silicon, or a non-Si semiconductor material).Moreover, while these calculations are shown for determining theapproximate optimal doping concentrations for uniformly doped baseregions, they may also serve as guides for determining the desiredin-situ-doped graded base profiles.

Assume P_(bl) is much less than 200 W/m²; e.g., assume: P_(bl)=( 1/1000)200=0.2 W/m²:

P _(bl)=0.2=6.93×10⁻⁴ R _(sb) →R _(sb)=288.6 Ω/square

Thus, R_(sb) should be less than 300 Ω/square in order for the baseohmic losses to be negligible.

R _(sb) ≈ρ/W→ρ≈R _(sb) ×W, where W is the silicon thickness.

For R_(sb)≈300 Ω/square, the maximum base resistivity values for variousprism silicon thicknesses are shown in FIG. 187. Note that the resultsare shown for a hexagonal-prism substrate with d of 100 microns and H of300 microns. Selective emitter sheet resistance is chosen to be on theorder of 100 Ω/square to 150 Ω/square; therefore, the emitter currentohmic losses are less than the base current ohmic losses and areexpected to be much less than 0.1% of the maximum cell power generation.

The hexagonal-prism 3-D TFSC substrates of this disclosure may utilizeperipheral thick silicon frames, both for added mechanical support andalso to facilitate formation of wrap-through or wrap-around emittercontact metallization (for ease of solar module assembly). FIGS. 161Aand 161B show the Y-Y cross-sectional views of a hexagonal-prism 3-DTFSC substrate using a thick silicon frame, before and after TFSCfabrication (relative substrate dimensions not shown to scale since inpractice the a large substrate such as a 200 mm×200 mm substrate hasthousands to millions of prism unit cells). The thick silicon frame maybe separately made from very low-cost silicon material (such asmetallurgical grade or reclaim silicon wafers). FIG. 188 shows variousschematic views 2230 of the thick silicon frame, the silicon frameslivers, and representative method to produce (e.g., cut) siliconslivers from very-low-cost round (e.g., reject silicon frommicroelectronics) or square-shaped (or rectangular) cast silicon (orreclaim Si) substrates. The slivers may be made of very low-costcrystalline or multicrystalline silicon such as metallurgical-grade castSi. A round 2232 or square-shaped 2234 silicon wafer (e.g., a 200 mm×200mm cast metallurgical-grade silicon substrate) may be used to producehundreds of silicon slivers 2236 by a cutting process such as lasercutting (four slivers used to make a thick silicon frame for a 3-D TFSCsubstrate by a welding process such as electron-beam welding).

These slivers 2236 may be used to make the thick silicon frames for thesubstrates shown in FIGS. 161A and 161B. The separately fabricated thicksilicon frame may then be integrally attached to the hexagonal-prism 3-DTFSC substrates, in embodiment before 3-D thin-film cell processing, byone of the following techniques: electron-beam welding at severalperipheral spots/junctions; attachment during the hexagonal prism 3-DTFSC substrate fabrication silicon deposition by placing the peripheralthick silicon frame on the template and allowing seamless attachment ofthe thick silicon frame to the 3-D TFSC substrate by the silicondeposition process; or a clean cured epoxy.

Top view 2240 shows a thick silicon frame to be fused to the 3-D TFSCsubstrate. The silicon frame thickness 2242 is roughly 50 to 500microns. There are welded (e.g., e-beam-welded) joints 2244 (four weldedjoints), where L 2246 is roughly 150 to 300 millimeters, and where W2248 is roughly 100 to 1000 microns. The slivers 2236 may also havethrough-holes (shown in view 2250) to help with thewrap-through/wrap-around emitter metallization contacts.

The main sources of efficiency drop from the cell-level efficiency tothe module-level efficiency (efficiency gap between the cells andmodules) in the state-of-the-art prior art commercial solar cells andmodules are: ohmic power losses due to the cell electrical contacts andinterconnects; ohmic power losses due to the cell-to-module electricalinterconnects as well as the module cell-to-cell electricalinterconnects (connecting the cells in series and/or in combinationseries/parallel); ratio of total active front cell area to the totalmodule front area (area cells-to-module area ratio); and frontside glasscover reflectance (and transmittance) losses. The combination of theselosses may cause a cell-to-module efficiency drop or gap of at leastroughly 4% to 7% in the state-of-the-art prior art solar modules. Thesolar cell and module assembly designs of this disclosure enable asubstantial reduction of this cell-to-module efficiency gap to belowroughly 1% to 3%. This is accomplished by: high ratio of active cellarea to module area (more than 99%); much reduced ohmic power losses dueto the cell and module electrical contact/interconnects (to well below1% to 2% due to the unique cell and module contact and interconnectdesigns and the printed-circuit board assembly features); andsubstantially reduced frontside cover optical reflectance/transmittancelosses due to glassless module passivation or textured-glass-coveredmodule designs (optical reflectance/transmittance losses reduced to lessthan 1%).

In summary, the disclosed subject matter provides solar modulestructures and methods for assembling solar module structures. The solarmodule structures comprise three-dimensional single and dual aperturethin-film solar cells arranged in solar module structures. Thethree-dimensional thin-film solar cell comprises a semiconductorsubstrate with self-aligned selective emitter regions and self-alignedbase diffusion regions. The solar module structures usingthree-dimensional dual-aperture thin-film solar cells may be used insolar glass. The solar module structures using three-dimensionalsingle-aperture thin-film solar cells may be used in building androoftop applications.

The foregoing description of the preferred embodiments is provided toenable any person skilled in the art to make or use the claimed subjectmatter. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without the use of theinnovative faculty. Thus, the claimed subject matter is not intended tobe limited to the embodiments shown herein but is to be accorded thewidest scope consistent with the principles and novel features disclosedherein.

1. A solar module structure, comprising: a top encapsulant layer; a plurality of three-dimensional thin-film solar cells, each thin-film solar cell comprising: a three-dimensional thin-film solar cell substrate comprising emitter junction regions and doped base regions; emitter metallization regions; and base metallization regions; wherein said three-dimensional thin-film solar cell substrate comprises a plurality of single-aperture unit cells; a printed circuit board; a rear encapsulant layer; and a protective back plate.
 2. The solar module structure of claim 1, further comprising a glass layer.
 3. The solar module structure of claim 2, wherein said glass layer comprises a separate glass cover.
 4. The solar module structure of claim 2, wherein said glass layer comprises a glass-like protective coating formed by a coating method.
 5. The solar module structure of claim 1, further comprising a top patterned cell interconnect layer.
 6. The solar module structure of claim 1, further comprising a rear patterned cell interconnect layer.
 7. The solar module structure of claim 1, wherein said protective back plate comprises a Tedlar protective back plate.
 8. The solar module structure of claim 1, wherein each three-dimensional thin-film solar cell further comprises a rear mirror.
 9. The solar module structure of claim 8, wherein said rear mirror comprises an integrated rear mirror.
 10. The solar module structure of claim 8, wherein said rear mirror comprises a detached rear mirror.
 11. The solar module structure of claim 1, wherein each three-dimensional thin-film solar cell further comprises a peripheral cell frame.
 12. The solar module structure of claim 11, wherein said peripheral cell frame may be used to produce wrap-through or wrap-around emitter metallization for making contacts to the cell emitter at the bottom of each three-dimensional thin-film solar cell.
 13. A method for assembling a solar module structure, comprising: patterning a frontside and a backside of a double-sided printed circuit board coated with metallic foils according to desired frontside and backside interconnect layouts; applying a first coating layer to the rear side of a plurality of three-dimensional thin-film solar cells, each three-dimensional thin-film solar cell comprising: a three-dimensional thin-film solar cell substrate comprising emitter junction regions and doped base regions; emitter metallization regions; and base metallization regions; wherein said three-dimensional thin-film solar cell substrate comprises a plurality of single-aperture unit cells; placing said plurality of three-dimensional thin-film solar cells on the frontside of said double-sided printed circuit board; preparing a solar module assembly, comprising: a glass layer; a top encapsulant layer; said plurality of three-dimensional thin-film solar cells on the frontside of said double-sided printed circuit board; a rear encapsulant layer; a protective back plate; and sealing and packaging said solar module assembly.
 14. The method for assembling a solar module structure of claim 13, wherein each three-dimensional thin-film solar cell further comprises a rear mirror.
 15. The method for assembling a solar module structure of claim 13, further comprising arranging a plurality of said solar module assemblies in series.
 16. The method for assembling a solar module structure of claim 13, further comprising arranging a plurality of said solar module assemblies in parallel.
 17. The method for assembling a solar module structure of claim 15, further comprising connecting at least one pair of module power output electrical leads.
 18. The method for assembling a solar module structure of claim 16, further comprising connecting at least one pair of module power output electrical leads.
 19. A solar module structure, comprising: a top glass plate; a plurality of three-dimensional thin-film solar cells, each three-dimensional thin-film solar cell comprising: a three-dimensional thin-film solar cell substrate comprising emitter junction regions and doped base regions; emitter metallization regions; and base metallization regions; wherein said three-dimensional thin-film solar cell substrate comprises a plurality of dual-aperture unit cells; a rear patterned cell interconnect layer; and a bottom glass plate.
 20. The solar module structure of claim 19, wherein each three-dimensional thin-film solar cell further comprises a peripheral cell frame.
 21. A method for assembling a solar module structure, comprising: applying a rear patterned cell interconnect layer to the rear side of a plurality of three-dimensional thin-film solar cells, each thin-film solar cell comprising: a three-dimensional thin-film solar cell substrate comprising emitter junction regions and doped base regions; emitter metallization regions; and base metallization regions; wherein said three-dimensional thin-film solar cell substrate comprises a plurality of dual-aperture unit cells; preparing a solar module assembly, comprising: a top glass plate; said plurality of three-dimensional thin-film solar cells with said rear patterned cell interconnect layer applied to the rear side; a bottom glass plate; sealing and packaging said solar module assembly.
 22. The method for assembling a solar module structure of claim 21, wherein said step of preparing a solar module assembly further comprises preparing a solar module assembly with a frame.
 23. The method for assembling a solar module structure of claim 21, wherein each three-dimensional thin-film solar cell further comprises a rear mirror.
 24. The method for assembling a solar module structure of claim 21, further comprising arranging a plurality of said solar module assemblies in series.
 25. The method for assembling a solar module structure of claim 21, further comprising arranging a plurality of said solar module assemblies in parallel.
 26. The method for assembling a solar module structure of claim 24, further comprising connecting at least one pair of module power output electrical leads.
 27. The method for assembling a solar module structure of claim 25, further comprising connecting at least one pair of module power output electrical leads.
 28. A solar module structure, comprising: a top protective layer; a plurality of three-dimensional thin-film solar cells, each thin-film solar cell comprising: a three-dimensional thin-film solar cell substrate comprising emitter and base regions; wherein said three-dimensional thin-film solar cell substrate comprises a plurality of unit cells; a printed circuit board; and a protective back plate.
 29. A solar module structure, comprising: a top glass plate; a plurality of three-dimensional thin-film solar cells, each thin-film solar cell comprising: a three-dimensional thin-film solar cell substrate comprising emitter and base regions; wherein said three-dimensional thin-film solar cell substrate comprises a plurality of unit cells; and a bottom glass plate.
 30. A solar module structure, comprising: a top protective layer; a plurality of three-dimensional thin-film solar cells, each thin-film solar cell comprising: a three-dimensional thin-film solar cell substrate comprising emitter and base regions; wherein said three-dimensional thin-film solar cell substrate comprises a plurality of unit cells; and a bottom protective layer. 